vhdx: Fix vhdx_co_create() return value

.bdrv_co_create() is supposed to return 0 on success, but vhdx could
return a positive value instead. Fix this.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>

block/vhdx.c

@@ -1951,7 +1951,7 @@ static int coroutine_fn vhdx_co_create(BlockdevCreateOptions *opts,
         goto delete_and_exit;
     }
 
-
+    ret = 0;
 delete_and_exit:
     blk_unref(blk);
     bdrv_unref(bs);