core: prevent disabling VirtIO-SCSI on edit VM

When a disk with VirtIO-SCSI interface is plugged to a VM,
disabling VirtIO-SCSI should be prevented.

Change-Id: Ic2e6c85a2df434de3180f19a32079f276458d372
Bug-Url: https://bugzilla.redhat.com/1014491
Signed-off-by: Daniel Erez <derez@redhat.com>