Update the check about coalesce

[yalzhang]

On latest rhel 8 system, when coalesce is not set, the default value for rx_frames change to n/a. While on rhel 9, it's 0. Both of them are expectable. Update the check.

[yalzhang]

Run on rhel 8, pass

 (1/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.nat_net.delete_iface_coalesce: STARTED
 (1/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.nat_net.delete_iface_coalesce: PASS (27.45 s)
 (2/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.nat_net.add_iface_coalesce: STARTED
 (2/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.nat_net.add_iface_coalesce: PASS (27.93 s)
 (3/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.nat_net.update_iface_coalesce: STARTED
 (3/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.nat_net.update_iface_coalesce: PASS (27.99 s)
 (4/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.linux_br.delete_iface_coalesce: STARTED
 (4/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.linux_br.delete_iface_coalesce: PASS (44.44 s)
 (5/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.linux_br.add_iface_coalesce: STARTED
 (5/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.linux_br.add_iface_coalesce: PASS (50.50 s)
 (6/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.linux_br.update_iface_coalesce: STARTED
 (6/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.linux_br.update_iface_coalesce: PASS (50.11 s)
 (7/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.ovs_br.delete_iface_coalesce: STARTED
 (7/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.ovs_br.delete_iface_coalesce: PASS (49.87 s)
 (8/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.ovs_br.add_iface_coalesce: STARTED
 (8/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.ovs_br.add_iface_coalesce: PASS (50.46 s)
 (9/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.ovs_br.update_iface_coalesce: STARTED
 (9/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.ovs_br.update_iface_coalesce: PASS (48.41 s)
RESULTS    : PASS 9 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB HTML   : /var/lib/avocado/job-results/job-2024-03-19T05.59-7ae194b/results.html
JOB TIME   : 381.79 s

Run on rhel 9, pass

 (1/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.nat_net.delete_iface_coalesce: STARTED
 (1/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.nat_net.delete_iface_coalesce: PASS (57.20 s)
 (2/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.nat_net.add_iface_coalesce: STARTED
 (2/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.nat_net.add_iface_coalesce: PASS (56.42 s)
 (3/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.nat_net.update_iface_coalesce: STARTED
 (3/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.nat_net.update_iface_coalesce: PASS (56.50 s)
 (4/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.linux_br.delete_iface_coalesce: STARTED
 (4/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.linux_br.delete_iface_coalesce: PASS (86.13 s)
 (5/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.linux_br.add_iface_coalesce: STARTED
 (5/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.linux_br.add_iface_coalesce: PASS (73.57 s)
 (6/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.linux_br.update_iface_coalesce: STARTED
 (6/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.linux_br.update_iface_coalesce: PASS (71.29 s)
 (7/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.ovs_br.delete_iface_coalesce: STARTED
 (7/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.ovs_br.delete_iface_coalesce: PASS (82.96 s)
 (8/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.ovs_br.add_iface_coalesce: STARTED
 (8/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.ovs_br.add_iface_coalesce: PASS (81.13 s)
 (9/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.ovs_br.update_iface_coalesce: STARTED
 (9/9) type_specific.io-github-autotest-libvirt.virtual_network.update_device.coalesce.ovs_br.update_iface_coalesce: PASS (85.42 s)
RESULTS    : PASS 9 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB HTML   : /root/avocado/job-results/job-2024-03-19T06.13-5fb7f1b/results.html
JOB TIME   : 659.59 s

[yalzhang]

@chloerh Please help to review, thank you!