core: send virtio-serial device to VDSM

Add virtio-serial to the list of devices the engine sends to VDSM

Change-Id: Ie99dafc1495f42ffcd76592dcd9130f12c6d61bd
Bug-Url: https://bugzilla.redhat.com/1028387
Signed-off-by: Arik Hadas <ahadas@redhat.com>