mirror: fix misleading comments

s/target bs/to_replace/, also we check to_replace bs is not
blocked in qmp_drive_mirror() not here

Signed-off-by: Changlong Xie <xiecl.fnst@cn.fujitsu.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 1466672241-22485-3-git-send-email-xiecl.fnst@cn.fujitsu.com
Signed-off-by: Jeff Cody <jcody@redhat.com>

block/mirror.c

@@ -775,7 +775,7 @@ static void mirror_complete(BlockJob *job, Error **errp)
         }
     }
 
-    /* check the target bs is not blocked and block all operations on it */
+    /* block all operations on to_replace bs */
     if (s->replaces) {
         AioContext *replace_aio_context;