block: another bdrv_append fix

bdrv_append must also copy open_flags to the top, because the snapshot
has BDRV_O_NO_BACKING set.  This causes interesting results if you
later use drive-reopen (not upstream) to reopen the image, and lose
the backing file in the process.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -977,6 +977,7 @@ void bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top)
     tmp = *bs_new;
 
     /* there are some fields that need to stay on the top layer: */
+    tmp.open_flags        = bs_top->open_flags;
 
     /* dev info */
     tmp.dev_ops           = bs_top->dev_ops;