qemu.tests: Add live snapshot chain cases

Add five live snapshot cases have similar process:
- create live snapshot when guest paused
- restore to previous state when guest become crash
- restore live snapshot after guest update
- restore live snapshot chain 2-restore to base
- restore to parent snapshot node

Signed-off-by: Yiqiao Pu <ypu@redhat.com>

Update interface to date

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Signed-off-by: Feng Yang <fyang@redhat.com>