Add missing image to instance booted from volume

When booting an instance from a volume, the image_ref for
the instance will be None. And the API do not get the image
information for the instance also. So when we calling the
related API, we get "" for instance.image.

Change-Id: I3c35ab1d7c8bcec551fb5d67d0b44418266b32a4
Closes-bug: 1317880
Merge "Add missing image to instance booted from volume"