job: Add job_sleep_ns()

There is nothing block layer specific about block_job_sleep_ns(), so
move the function to Job.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
diff --git a/block/commit.c b/block/commit.c
index 2fbc310..1c6cb6c 100644
--- a/block/commit.c
+++ b/block/commit.c
@@ -172,7 +172,7 @@
         /* Note that even when no rate limit is applied we need to yield
          * with no pending I/O here so that bdrv_drain_all() returns.
          */
-        block_job_sleep_ns(&s->common, delay_ns);
+        job_sleep_ns(&s->common.job, delay_ns);
         if (job_is_cancelled(&s->common.job)) {
             break;
         }