We are going to remove bs->job pointer. Drop it's usage in
blk_iostatus_reset.

blk_iostatus_reset() has only two callers:

1. blk_attach_dev(). This doesn't have anything to do with jobs and
    attaching a new guest device won't solve any problem the job
    encountered, so no reason to reset the iostatus for the job.

2. qmp_cont(). This resets the iostatus for everything. We can just
    call block_job_iostatus_reset() for all block jobs instead of going
    through BlockBackend.
Suggested-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>