block-stream is the only block-job, that reasonably use BlockJob.blk.
We are going to drop BlockJob.blk soon. So, let block-stream have own
blk.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Nikita Lapshin <nikita.lapshin@virtuozzo.com>