storage: improve allocation stats reported on gluster files

storage: improve allocation stats reported on gluster files

We already had code for handling allocation different than
capacity for sparse files; we just had to wire it up to be
used when inspecting gluster images.

* src/storage/storage_backend.c
(virStorageBackendUpdateVolTargetInfoFD): Handle no fd.
* src/storage/storage_backend_gluster.c
(virStorageBackendGlusterRefreshVol): Handle sparse files.

Signed-off-by: Eric Blake <eblake@redhat.com>