core: clone VM from snapshot - validate VirtIO-SCSI

Validate whether VirtIO-SCSI can be disabled when cloning
a VM from snapshot (similar to the validation on UpdateVmCommand):
* Added a validation method to VmValidator.
* Added appropriate tests.

Change-Id: I31d4ebe3a964fb54cb1a905460b9da785a746419
Bug-Url: https://bugzilla.redhat.com/1133475
Signed-off-by: Daniel Erez <derez@redhat.com>