xen_disk: simplify blk_disconnect with refcnt

We call bdrv_attach_dev when initializing whether or not bs is created
locally, so call bdrv_detach_dev and let the refcnt handle the
lifecycle.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

hw/block/xen_disk.c

@@ -824,6 +824,9 @@ static int blk_connect(struct XenDevice *xendev)
         /* setup via qemu cmdline -> already setup for us */
         xen_be_printf(&blkdev->xendev, 2, "get configured bdrv (cmdline setup)\n");
         blkdev->bs = blkdev->dinfo->bdrv;
+        /* blkdev->bs is not create by us, we get a reference
+         * so we can bdrv_unref() unconditionally */
+        bdrv_ref(blkdev->bs);
     }
     bdrv_attach_dev_nofail(blkdev->bs, blkdev);
     blkdev->file_size = bdrv_getlength(blkdev->bs);
@@ -922,12 +925,8 @@ static void blk_disconnect(struct XenDevice *xendev)
     struct XenBlkDev *blkdev = container_of(xendev, struct XenBlkDev, xendev);
 
     if (blkdev->bs) {
-        if (!blkdev->dinfo) {
-            /* close/delete only if we created it ourself */
-            bdrv_close(blkdev->bs);
-            bdrv_detach_dev(blkdev->bs, blkdev);
-            bdrv_unref(blkdev->bs);
-        }
+        bdrv_detach_dev(blkdev->bs, blkdev);
+        bdrv_unref(blkdev->bs);
         blkdev->bs = NULL;
     }
     xen_be_unbind_evtchn(&blkdev->xendev);