Fix bugs for guestfish

Fix some bugs for libguestfs tests:
1. Virtio disks in vm will be sdx, so to check it in vm, we should set it back to vdx.
2.Check vm state before testing, otherwise, tests will be end abnormally.
3.Volume we get from virt-commands my be not in same ruler in vm,
   virt: /dev/volumegroup/volumename
   vm:/dev/mapper/volumegroup-volumename
so check only volume name to avoid this problem
4.Pass correct vm for cloned vm: new_vm = vt.newvm

Relative PR: https://github.com/autotest/virt-test/pull/1778