qom: use ERRP_GUARD in user_creatable_complete

With error_propagate, the stack trace from any error_abort/fatal
usage will start from the error_propagate() call, which is largely
useless. Using ERRP_GUARD ensures the stack trace starts from
the origin that reported the error.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

qom/object_interfaces.c

@@ -24,13 +24,12 @@
 bool user_creatable_complete(UserCreatable *uc, Error **errp)
 {
     UserCreatableClass *ucc = USER_CREATABLE_GET_CLASS(uc);
-    Error *err = NULL;
+    ERRP_GUARD();
 
     if (ucc->complete) {
-        ucc->complete(uc, &err);
-        error_propagate(errp, err);
+        ucc->complete(uc, errp);
     }
-    return !err;
+    return !*errp;
 }
 
 bool user_creatable_can_be_deleted(UserCreatable *uc)