mirror: Confirm we're quiesced only if the job is paused or cancelled

mirror: Confirm we're quiesced only if the job is paused or cancelled

While child_job_drained_begin() calls to job_pause(), the job doesn't
actually transition between states until it runs again and reaches a
pause point. This means bdrv_drained_begin() may return with some jobs
using the node still having 'busy == true'.

As a consequence, block_job_detach_aio_context() may get into a
deadlock, waiting for the job to be actually paused, while the coroutine
servicing the job is yielding and doesn't get the opportunity to get
scheduled again. This situation can be reproduced by issuing a
'block-commit' immediately followed by a 'device_del'.

To ensure bdrv_drained_begin() only returns when the jobs have been
paused, we change mirror_drained_poll() to only confirm it's quiesced
when job->paused == true and there aren't any in-flight requests, except
if we reached that point by a drained section initiated by the
mirror/commit job itself.

The other block jobs shouldn't need any changes, as the default
drained_poll() behavior is to only confirm it's quiesced if the job is
not busy or completed.

Signed-off-by: Sergio Lopez <slp@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>