protocol: Don't use rename as a variable name

That causes shadowing errors on older compilers.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

src/remote/remote_protocol.x

@@ -3237,7 +3237,7 @@ struct remote_domain_rename_args {
 };
 
 struct remote_domain_rename_ret {
-    int rename;
+    int retcode;
 };
 
 /*----- Protocol. -----*/

src/remote_protocol-structs

@@ -2690,7 +2690,7 @@ struct remote_domain_rename_args {
         u_int                      flags;
 };
 struct remote_domain_rename_ret {
-        int                        rename;
+        int                        retcode;
 };
 enum remote_procedure {
         REMOTE_PROC_CONNECT_OPEN = 1,