restapi: optional disks list in create snapshot

BackendSnapshotsResource:
* set parameters' disks list when needed (doAdd method).
* added mapDisks method.

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

Change-Id: Idabf1b152136ec92bb703903ba40e695be4d57aa
Signed-off-by: Daniel Erez <derez@redhat.com>