StorPool: really detach volumes and snapshots after copying to/from an image

When copying a StorPool-backed volume to a Glance image (if Cinder is not used as the backing store for Glance), a temporary StorPool snapshot is created for the volume. This snapshot ought then to be deleted, but the StorPool query to delete it will always fail since it is still marked as attached to the Cinder volume host.

Similarly, when copying a Glance image to a StorPool volume (again, if Cinder is not used as the backing store for Glance), the volume itself is attached to the Cinder host and is never really detached afterwards.

Since StorPool has always transparently supported volumes and snapshots attached to multiple hosts (especially if no processes on these hosts attempt to use them, since everyone has forgotten about them), this only results in stale attachments and sometimes stale StorPool snapshots; there are no ill effects on the operation of the StorPool cluster or the Cinder volume service. Still, there is no reason for these attachments and snapshots to persist.