libvirt.test: Skip test if current large than max vcpu

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Actually, there is already such kind case in this test:
`type_specific.io-github-autotest-libvirt.virsh.setvcpus.error_test.invalid_vcpu_count_is_3`
and this NAError can mention user to set domain's max vcpu count large than 1.