Fix: Unshelving an instance uses original image

The driver uses the property image_ref to boot the
the instance.
This patch updates the image_ref property with the
shelved_image_id stored during the shelving.

Change-Id: I3bba0a230044613e07122a6d122597e5b8d43438
Closes-Bug: #1257594
Merge "Fix: Unshelving an instance uses original image"