We do add COW child to the node.  In future we are going to forbid
adding COW child to the node that doesn't support backing. So, fix it
here now.

Don't worry about setting bs->backing itself: it further commit we'll
update the block-layer to automatically set/unset this field in generic
code.

Signed-off-by: Vladimir Sementsov-Ogievskiy <v.sementsov-og@mail.ru>
---
 tests/unit/test-bdrv-drain.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tests/unit/test-bdrv-drain.c b/tests/unit/test-bdrv-drain.c
index 36be84ae55..23d425a494 100644
--- a/tests/unit/test-bdrv-drain.c
+++ b/tests/unit/test-bdrv-drain.c
@@ -1948,6 +1948,7 @@ static void coroutine_fn bdrv_replace_test_co_drain_end(BlockDriverState *bs)
 static BlockDriver bdrv_replace_test = {
     .format_name            = "replace_test",
     .instance_size          = sizeof(BDRVReplaceTestState),
+    .supports_backing       = true,
 
     .bdrv_close             = bdrv_replace_test_close,
     .bdrv_co_preadv         = bdrv_replace_test_co_preadv,
-- 
2.35.1