qemu-img rebase: Open new backing file read-only

We never write to a backing file, so opening rw is useless. It just means that
you can't rebase on top of a file for which you don't have write permissions.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-img.c

@@ -1286,7 +1286,7 @@ static int img_rebase(int argc, char **argv)
         }
 
         bs_new_backing = bdrv_new("new_backing");
-        ret = bdrv_open(bs_new_backing, out_baseimg, BDRV_O_FLAGS | BDRV_O_RDWR,
+        ret = bdrv_open(bs_new_backing, out_baseimg, BDRV_O_FLAGS,
                         new_backing_drv);
         if (ret) {
             error("Could not open new backing file '%s'", out_baseimg);