vdi: Fix vdi_co_do_create() return value

.bdrv_co_create() is supposed to return 0 on success, but vdi 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/vdi.c

@@ -865,6 +865,7 @@ static int coroutine_fn vdi_co_do_create(BlockdevCreateOptions *create_options,
         }
     }
 
+    ret = 0;
 exit:
     blk_unref(blk);
     bdrv_unref(bs_file);