conf: virDomainDefMaybeAddController tweak return code

Previously the function returned either -1 in case of an error or 0 on
success. However, we should also distinguish between a case we
successfully added a controller and a case there wasn't a need to add any
controller

src/conf/domain_conf.c

@@ -12416,7 +12416,7 @@ virDomainDefMaybeAddController(virDomainDefPtr def,
         return -1;
     }
 
-    return 0;
+    return 1;
 }