block: Add reference parameter to bdrv_open()

Allow bdrv_open() to handle references to existing block devices just as
bdrv_file_open() is already capable of.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/qemu-nbd.c b/qemu-nbd.c
index 0cf123c..711162c 100644
--- a/qemu-nbd.c
+++ b/qemu-nbd.c
@@ -597,7 +597,7 @@
 
     bs = bdrv_new("hda");
     srcpath = argv[optind];
-    ret = bdrv_open(&bs, srcpath, NULL, flags, drv, &local_err);
+    ret = bdrv_open(&bs, srcpath, NULL, NULL, flags, drv, &local_err);
     if (ret < 0) {
         errno = -ret;
         err(EXIT_FAILURE, "Failed to bdrv_open '%s': %s", argv[optind],