blockjob: Use bdrv_coroutine_enter to start coroutine

Resuming and especially starting of the block job coroutine, could be issued in
the main thread.  However the coroutine's "home" ctx should be set to the same
context as job->blk. Use bdrv_coroutine_enter to ensure that.
Signed-off-by: NFam Zheng <famz@redhat.com>
Acked-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>

blockjob.c

@@ -290,7 +290,7 @@ void block_job_start(BlockJob *job)
     job->pause_count--;
     job->busy = true;
     job->paused = false;
-    qemu_coroutine_enter(job->co);
+    bdrv_coroutine_enter(blk_bs(job->blk), job->co);
 }
 
 void block_job_ref(BlockJob *job)
@@ -532,7 +532,7 @@ void block_job_user_resume(BlockJob *job)
 void block_job_enter(BlockJob *job)
 {
     if (job->co && !job->busy) {
-        qemu_coroutine_enter(job->co);
+        bdrv_coroutine_enter(blk_bs(job->blk), job->co);
     }
 }