Fail to snapshot PowerVC image

When powervc driver found there is no or deleted image associated
during sync instances, it will set a default image which causes
exploiter crash, this patch will keep original image_ref when no
related image found.

Change-Id: I191d31cfdecaedd22b449728be732ca54057c5f9
Closes-Bug: 1404777
Merge "Fail to snapshot PowerVC image"