job: Add job_yield()

This moves block_job_yield() to the Job layer.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
diff --git a/block/mirror.c b/block/mirror.c
index c63cf7c..687f955 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -731,7 +731,7 @@
         block_job_event_ready(&s->common);
         s->synced = true;
         while (!job_is_cancelled(&s->common.job) && !s->should_complete) {
-            block_job_yield(&s->common);
+            job_yield(&s->common.job);
         }
         s->common.job.cancelled = false;
         goto immediate_exit;