virt: add Multi-host migration test for check time drift.

1) Syncs hosts time via timesync command.
2) After first boot of vms test check time on vms
   if time difference from host is greater than
   time_diff_limit it writes warning to log.
3) Migrates machine between hosts and compares
   time differences on hosts and guests after migration.
  - If difference is greater than time_diff_limit raise error.

Signed-off-by: Jiří Župka <jzupka@redhat.com>