block: ensure bdrv_drain_all() works during bdrv_delete()

In bdrv_delete() make sure to call bdrv_make_anon() *after* bdrv_close()
so that the device is still seen by bdrv_drain_all() when iterating
bdrv_states.

Cc: qemu-stable@nongnu.org
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block.c

diff --git a/block.c b/block.c
index 01b66d8..d5ce8d3 100644
--- a/block.c
+++ b/block.c
@@ -1606,11 +1606,11 @@
     assert(!bs->job);
     assert(!bs->in_use);
 
+    bdrv_close(bs);
+
     /* remove from list, if necessary */
     bdrv_make_anon(bs);
 
-    bdrv_close(bs);
-
     g_free(bs);
 }