blockjob: implement and use block_job_get_aio_context

We are going to drop BlockJob.blk. So let's retrieve block job context
from underlying job instead of main node.

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

blockjob.c

@@ -547,3 +547,8 @@ BlockErrorAction block_job_error_action(BlockJob *job, BlockdevOnError on_err,
     }
     return action;
 }
+
+AioContext *block_job_get_aio_context(BlockJob *job)
+{
+    return job->job.aio_context;
+}