core: VmRunHandler: snapshot check on diskless VMs

Since oVirt 3.1 supports snapshoting a diskless VM, VmRunHandler should
make sure a VM is not locked or during a snapshot when attempting to
run it even if it doesn't have any disks.

Change-Id: I232971b0d92aed1266d51ef89b2ae8175f08fbb2
Signed-off-by: Allon Mureinik <amureini@redhat.com>