core: GetActiveVmsByStorageDomainId - consider disk snapshots

GetActiveVmsByStorageDomainId should also returns vms that has plugged
disk snapshots on the domain.

Change-Id: I910d61ae2cf33782eac2ca53d92deb03a29f8531
Signed-off-by: Liron Aravot <laravot@redhat.com>