qobject: Replace qobject_incref/QINCREF qobject_decref/QDECREF

Now that we can safely call QOBJECT() on QObject * as well as its
subtypes, we can have macros qobject_ref() / qobject_unref() that work
everywhere instead of having to use QINCREF() / QDECREF() for QObject
and qobject_incref() / qobject_decref() for its subtypes.

The replacement is mechanical, except I broke a long line, and added a
cast in monitor_qmp_cleanup_req_queue_locked(). Unlike
qobject_decref(), qobject_unref() doesn't accept void *.

Note that the new macros evaluate their argument exactly once, thus no
need to shout them.

Backports commit cb3e7f08aeaab0ab13e629ce8496dca150a449ba from qemu

qemu/scripts/qapi/events.py

@@ -142,7 +142,7 @@ out:
 ''')
     ret += mcgen('''
     error_propagate(errp, err);
-    QDECREF(qmp);
+    qobject_unref(qmp);
 }
 ''')
     return ret