qobject: Add a special null QObject

I'm going to fix the JSON parser to recognize null.  The obvious
representation of JSON null as (QObject *)NULL doesn't work, because
the parser already uses it as an error value.  Perhaps we should
change it to free NULL for null, but that's more than I can do right
now.  Create a special null QObject instead.

The existing QDict, QList, and QString all represent something that
is a pointer in C and could therefore be associated with NULL.  But
right now, all three of these sub-types are always non-null once
created, so the new null sentinel object is intentionally unrelated
to them.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

qobject/qnull.c

@@ -0,0 +1,29 @@
+/*
+ * QNull
+ *
+ * Copyright (C) 2015 Red Hat, Inc.
+ *
+ * Authors:
+ *  Markus Armbruster <armbru@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU LGPL, version 2.1
+ * or later.  See the COPYING.LIB file in the top-level directory.
+ */
+
+#include "qemu-common.h"
+#include "qapi/qmp/qobject.h"
+
+static void qnull_destroy_obj(QObject *obj)
+{
+    assert(0);
+}
+
+static const QType qnull_type = {
+    .code = QTYPE_QNULL,
+    .destroy = qnull_destroy_obj,
+};
+
+QObject qnull_ = {
+    .type = &qnull_type,
+    .refcnt = 1,
+};