core: verify VirtIO-SCSI enabled on DiskValidator

* DiskValidator -> isVirtIoScsiValid:
  VirtIO-SCSI interface is valid only when VirtIO-SCSI is enabled
  for the VM (i.e. a controller is attached to the VM).
* Altered relevant commands/tests accordingly.

Change-Id: Iff20860a8462df49afbc21a66faf2614904a7a72
Bug-Url: https://bugzilla.redhat.com/1014491
Signed-off-by: Daniel Erez <derez@redhat.com>