qmp-input-visitor: Favor new visit_free() function

Now that we have a polymorphic visit_free(), we no longer need
qmp_input_visitor_cleanup(); which in turn means we no longer
need to return a subtype from qmp_input_visitor_new() nor a
public upcast function.

Generated code changes to qmp-marshal.c look like:

|@@ -52,11 +52,10 @@ void qmp_marshal_add_fd(QDict *args, QOb
| {
|     Error *err = NULL;
|     AddfdInfo *retval;
|-    QmpInputVisitor *qiv = qmp_input_visitor_new(QOBJECT(args), true);
|     Visitor *v;
|     q_obj_add_fd_arg arg = {0};
|
|-    v = qmp_input_get_visitor(qiv);
|+    v = qmp_input_visitor_new(QOBJECT(args), true);
|     visit_start_struct(v, NULL, NULL, 0, &err);
|     if (err) {
|         goto out;

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1465490926-28625-8-git-send-email-eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
diff --git a/qapi/qmp-input-visitor.c b/qapi/qmp-input-visitor.c
index 41b7f87..21edb39 100644
--- a/qapi/qmp-input-visitor.c
+++ b/qapi/qmp-input-visitor.c
@@ -373,25 +373,15 @@
     *present = true;
 }
 
-Visitor *qmp_input_get_visitor(QmpInputVisitor *v)
-{
-    return &v->visitor;
-}
-
 static void qmp_input_free(Visitor *v)
 {
     QmpInputVisitor *qiv = to_qiv(v);
 
-    qmp_input_visitor_cleanup(qiv);
+    qobject_decref(qiv->root);
+    g_free(qiv);
 }
 
-void qmp_input_visitor_cleanup(QmpInputVisitor *v)
-{
-    qobject_decref(v->root);
-    g_free(v);
-}
-
-QmpInputVisitor *qmp_input_visitor_new(QObject *obj, bool strict)
+Visitor *qmp_input_visitor_new(QObject *obj, bool strict)
 {
     QmpInputVisitor *v;
 
@@ -419,5 +409,5 @@
     v->root = obj;
     qobject_incref(obj);
 
-    return v;
+    return &v->visitor;
 }