core: Set images with Storage Pool for images already on target.

Setting images with storage pool id only for images which are already on
the Storage Domain, since importing a VM from configuration through the REST
does not oblegate an initialized Storage Domain

Change-Id: I8546a2ac47dab4208da0df4c2cd7bb92fdde1d40
Bug-Url: https://bugzilla.redhat.com/1124879
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>