qcow2: Fix data file error condition in qcow2_co_create()

We were trying to check whether bdrv_open_blockdev_ref() returned
success, but accidentally checked the wrong variable. Spotted by
Coverity (CID 1399703).
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NStefano Garzarella <sgarzare@redhat.com>

block/qcow2.c

@@ -3073,7 +3073,7 @@ qcow2_co_create(BlockdevCreateOptions *create_options, Error **errp)
             goto out;
         }
         data_bs = bdrv_open_blockdev_ref(qcow2_opts->data_file, errp);
-        if (bs == NULL) {
+        if (data_bs == NULL) {
             ret = -EIO;
             goto out;
         }