virtual_disk: fix snapshot issue for ovmf guest

[meinaLi]

There are two problems in virtual_disks_multidisks.py script:

1. The os xml will change to seabios guest when we testing ovmf guest because of incorrectos xml configuration
2. Internal snapshot is not supportted in ovmf guest.

Fix the above problems by adding os_firmware attribute and skip internal snapshot test for ovmf guest.

[meinaLi]

# avocado run --vt-type libvirt --test-runner=runner --vt-machine-type q35 virtual_disks.multidisks.coldplug.single_disk_test.disk_option_bootdisk.disk_virtio_bootorder_snapshot
JOB ID     : bb18d4040f97af2d43a73e3a607705a154e833eb
JOB LOG    : /var/lib/avocado/job-results/job-2024-04-18T03.26-bb18d40/job.log
 (1/1) type_specific.io-github-autotest-libvirt.virtual_disks.multidisks.coldplug.single_disk_test.disk_option_bootdisk.disk_virtio_bootorder_snapshot: PASS (131.25 s)
RESULTS    : PASS 1 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB HTML   : /var/lib/avocado/job-results/job-2024-04-18T03.26-bb18d40/results.html
JOB TIME   : 132.82 s
# avocado run --vt-type libvirt --test-runner=runner --vt-machine-type q35 virtual_disks..multidisks..disk_virtio_snapshot
JOB ID     : 1dc6d367c94b24ea1f93e2efd6f7cc1b43d9444d
JOB LOG    : /var/lib/avocado/job-results/job-2024-04-18T03.31-1dc6d36/job.log
 (1/7) type_specific.io-github-autotest-libvirt.virtual_disks.multidisks.coldplug.single_disk_test.disk_option_bootdisk.disk_virtio_snapshot.disk_snapshot_external.normal_test: PASS (110.55 s)
 (2/7) type_specific.io-github-autotest-libvirt.virtual_disks.multidisks.coldplug.single_disk_test.disk_option_bootdisk.disk_virtio_snapshot.disk_snapshot_external.ahci_disk_test: PASS (110.55 s)
 (3/7) type_specific.io-github-autotest-libvirt.virtual_disks.multidisks.coldplug.single_disk_test.disk_option_bootdisk.disk_virtio_snapshot.disk_snapshot_external.error_test: PASS (149.76 s)
 (4/7) type_specific.io-github-autotest-libvirt.virtual_disks.multidisks.coldplug.single_disk_test.disk_option_bootdisk.disk_virtio_snapshot.disk_snapshot_external.error_disk_read_only_test: PASS (171.96 s)
 (5/7) type_specific.io-github-autotest-libvirt.virtual_disks.multidisks.coldplug.single_disk_test.disk_option_bootdisk.disk_virtio_snapshot.disk_snapshot_no.error_test1: PASS (107.51 s)
 (6/7) type_specific.io-github-autotest-libvirt.virtual_disks.multidisks.coldplug.single_disk_test.disk_option_bootdisk.disk_virtio_snapshot.disk_snapshot_no.error_test2: PASS (167.32 s)
 (7/7) type_specific.io-github-autotest-libvirt.virtual_disks.multidisks.coldplug.single_disk_test.disk_option_bootdisk.disk_virtio_snapshot.disk_snapshot_no.error_disk_read_only_test: PASS (140.29 s)
RESULTS    : PASS 7 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB HTML   : /var/lib/avocado/job-results/job-2024-04-18T03.31-1dc6d36/results.html
JOB TIME   : 959.80 s