Change variants name to avoid avocado list test case failure

[liang-cong-red-hat]

Before the fix:
command shows no result
# avocado list --vt-type libvirt memory

After the fix:

# avocado list --vt-type libvirt memory
No python imaging library installed. Screendump and Windows guest BSOD detection are disabled. In order to enable it, please install python-imaging or the equivalent for your distro.
No python imaging library installed. PPM image conversion to JPEG disabled. In order to enable it, please install python-imaging or the equivalent for your distro.
avocado-vt type_specific.io-github-autotest-libvirt.memory.devices.invalid_nvdimm.with_numa.exceed_slot
avocado-vt type_specific.io-github-autotest-libvirt.memory.devices.invalid_nvdimm.with_numa.max_addr
avocado-vt type_specific.io-github-autotest-libvirt.memory.devices.invalid_nvdimm.with_numa.unexisted_node
avocado-vt type_specific.io-github-autotest-libvirt.memory.devices.invalid_nvdimm.with_numa.unexisted_path
...

Rerun test results:

# avocado run --vt-type libvirt --vt-machine-type q35 --test-runner=runner memory.backing.nodeset
(1/4) type_specific.io-github-autotest-libvirt.memory.backing.nodeset.with_numa.nodeset_0.4k.2048: PASS (36.50 s)
 (2/4) type_specific.io-github-autotest-libvirt.memory.backing.nodeset.with_numa.nodeset_not_exist.4k.2048: PASS (8.17 s)
 (3/4) type_specific.io-github-autotest-libvirt.memory.backing.nodeset.without_numa.nodeset_0.4k.2048: PASS (34.07 s)
 (4/4) type_specific.io-github-autotest-libvirt.memory.backing.nodeset.without_numa.nodeset_not_exist.4k.2048: PASS (7.78 s)

[dzhengfy]

LGTM