qapi: check invalid arguments on no-args commands

The generated marshal functions do not visit arguments from commands
that take no arguments. Thus they fail to catch invalid
members. Visit the arguments, if provided, to throw an error in case of
invalid members.

Currently, qmp_check_client_args() checks for invalid arguments and
correctly catches this case. When switching to qmp_dispatch() we want to
keep that behaviour. The commands using 'O' may have arbitrary
arguments, and must have 'gen': false in the qapi schema to skip the
generated checks.

Old/new diff:
 void qmp_marshal_stop(QDict *args, QObject **ret, Error **errp)
 {
     Error *err = NULL;
+    Visitor *v = NULL;

-    (void)args;
+    if (args) {
+        v = qmp_input_visitor_new(QOBJECT(args), true);
+        visit_start_struct(v, NULL, NULL, 0, &err);
+        if (err) {
+            goto out;
+        }
+
+        if (!err) {
+            visit_check_struct(v, &err);
+        }
+        visit_end_struct(v, NULL);
+        if (err) {
+            goto out;
+        }
+    }

     qmp_stop(&err);
+
+out:
     error_propagate(errp, err);
+    visit_free(v);
+    if (args) {
+        v = qapi_dealloc_visitor_new();
+        visit_start_struct(v, NULL, NULL, 0, NULL);
+
+        visit_end_struct(v, NULL);
+        visit_free(v);
+    }
 }

The new code closely resembles code for a command with arguments.
Differences:
- the visit of the argument and its cleanup struct don't visit any
  members (because there are none).
- the visit of the argument struct and its cleanup are conditional.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20160912091913.15831-14-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
