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>