Comment 4 for bug 1732428

Now I'm seeing this:

https://github.com/openstack/nova/blob/b6a245f0425a07be3871a976952646d2bdd44533/nova/compute/manager.py#L4797

So apparently we set the instance.image_ref back to the original image_ref because after we successfully spawn the guest during unshelve, we delete the snapshot image reference for some reason, I have no idea why.