BlockDriverState members change_cb and change_opaque are initially
null.  The device model may set them, with bdrv_set_change_cb().  If
the device model gets detached (hot unplug), they're left dangling.
Only safe because device hot unplug automatically destroys the
BlockDriverState.  But that's a questionable feature, best not to rely
on it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>