This function is called in two different places:
- timer callback, which does not take the graph rdlock.
- bdrv_qed_drain_begin(), which is .bdrv_drain_begin()
  callback documented as function that does not take the lock.

Since it calls recursive functions that traverse the
graph, we need to protect them with the graph rdlock.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230203152202.49054-7-kwolf@redhat.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>