Storage resource usage is currently tracked for disks and snapshots in the virtual_provisioning_resource and virtual_provisioning_collection database tables. When an image is made from a snapshot, it shares the same backend (a crucible read-only region) so we do not count images towards resource usage. However, a snapshot can be deleted while the image created from it lives on. When that happens, we decrement the snapshot usage from the database table. The image resource usage is "leaked" in this case. We'll need a way to account for image usage when their snapshot origins are deleted.