restapi: support custom preview snapshot

Accept optional 'restore_memory' and 'disks' tags
in preview_snapshot action.

Preview Snapshot: POST /api/vms/{vm_id}/preview_snapshot

 <action>
   <snapshot id="{snapshot_id}"/>
   <restore_memory>true|false</restore_memory>
   <disks>
     <disk id="{disk_id}">
       <image_id>{image_id}</image_id>
       <snapshot id="{snapshot_id}"/>
     </disk>
   </disks>
 </action>

Feature-Page: http://www.ovirt.org/Features/Single_Disk_Snapshot#REST-API

Change-Id: I89d004dec75aa23c6a7105ddb064e93d7f4652ed
Bug-Url: https://bugzilla.redhat.com/1095494
Signed-off-by: Daniel Erez <derez@redhat.com>