With dataplane, the ioeventfd events could be dispatched after
mirror_run releases the dirty bitmap, but before mirror_exit actually
does the device switch, because the iothread will still be running, and
it will cause silent data loss.
Fix this by adding a bdrv_drained_begin/end pair around the window, so
that no new external request will be handled.
Signed-off-by: Fam Zheng <famz@redhat.com> Signed-off-by: Jeff Cody <jcody@redhat.com>