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.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180419150145.24795-4-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Rebased, semantic conflict resolved, commit message improved]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
diff --git a/tests/check-qobject.c b/tests/check-qobject.c
index 7629b80..5cb08fc 100644
--- a/tests/check-qobject.c
+++ b/tests/check-qobject.c
@@ -80,7 +80,7 @@
 
     va_start(ap, _);
     while ((obj = va_arg(ap, QObject *)) != NULL) {
-        qobject_decref(obj);
+        qobject_unref(obj);
     }
     va_end(ap);
 }