add mem file for vcpu_misc: snapshot scenario

[nanli1]

add --memspec MEM_FILE when virsh snapshot-create-as Signed-off-by: nanli [email protected]

avocado run --vt-type libvirt --test-runner=runner --vt-machine-type q35 vcpu_misc.positive_test.snapshot 
 (1/1) type_specific.io-github-autotest-libvirt.vcpu_misc.positive_test.snapshot: PASS (75.38 s)

Other related

avocado run --vt-type libvirt --test-runner=runner --vt-machine-type q35 vcpu_misc

 (1/8) type_specific.io-github-autotest-libvirt.vcpu_misc.positive_test.snapshot: PASS (82.98 s)
 (2/8) type_specific.io-github-autotest-libvirt.vcpu_misc.positive_test.vendor_id: PASS (53.61 s)
 (3/8) type_specific.io-github-autotest-libvirt.vcpu_misc.positive_test.with_topology.vcpus_max_equal_topology: PASS (130.55 s)
 (4/8) type_specific.io-github-autotest-libvirt.vcpu_misc.positive_test.mode_max: PASS (53.86 s)
 (5/8) type_specific.io-github-autotest-libvirt.vcpu_misc.positive_test.managedsave_restore: PASS (60.59 s)
 (6/8) type_specific.io-github-autotest-libvirt.vcpu_misc.positive_test.with_maxphysaddr: PASS (54.29 s)
 (7/8) type_specific.io-github-autotest-libvirt.vcpu_misc.negative_test.with_topology.vcpus_max_exceed_topology: PASS (27.85 s)
 (8/8) type_specific.io-github-autotest-libvirt.vcpu_misc.negative_test.with_topology.vcpus_max_less_than_topology: PASS (27.56 s)