libxl: fix return code in qmp_synchronous_send

Use error code from libxl namespace, a plain -1 is not valid in this context.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Wei Liu <wei.liu2@citrix.com>

tools/libxl/libxl_qmp.c

@@ -627,7 +627,7 @@ static int qmp_synchronous_send(libxl__qmp_handler *qmp, const char *cmd,
 
     id = qmp_send(qmp, cmd, args, callback, opaque, &context);
     if (id <= 0) {
-        return -1;
+        return ERROR_FAIL;
     }
     qmp->wait_for_id = id;