If main job coroutine called job_yield (while some background process
is in progress), we should give it a chance to call job_pause_point().
It will be used in backup, when moved on async block-copy.

Note, that job_user_pause is not enough: we want to handle
child_job_drained_begin() as well, which call job_pause().

Still, if job is already in job_do_yield() in job_pause_point() we
should not enter it.

iotest 109 output is modified: on stop we do bdrv_drain_all() which now
triggers job pause immediately (and pause after ready is standby).

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210116214705.822267-10-vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>