storage: Don't adjust pool alloc/avail values for disk backend

storage: Don't adjust pool alloc/avail values for disk backend

Commit id '2ac0e647' for https://bugzilla.redhat.com/show_bug.cgi?id=1206521
was meant to be a generic check for the CreateVol, CreateVolFrom, and
DeleteVol paths to check if the storage backend's changed the pool's view
of allocation or available values.

Unfortunately as it turns out this caused a side effect when the disk backend
created an extended partition there would be no actual storage removed from
the pool, thus the changes would not find any change in allocation or
available and incorrectly update the pool values using the size of the
extended partition. A subsequent refresh of the pool would reset the
values appropriately.

This patch modifies those checks in order to specifically not update the
pool allocation and available for only the disk backend rather than be
generic before and after checks.