restapi: optional disks list for snapshot restore

BackendSnapshotResource -> restore:
set parameters' disks list when needed (doAdd method).

Feature: http://www.ovirt.org/Features/Single_Disk_Snapshot

Change-Id: I82ed06edf8c82522585d78c02953cae6831b3a4a
Signed-off-by: Daniel Erez <derez@redhat.com>