qobject: Update coccinelle script to catch Q{INC, DEC}REF

The recent commit b097efc0 used qobject_decref(QOBJECT(E)), even
though we already have QDECREF(E) for that purpose.  We can update
our coccinelle script to catch any future relapses; with that in
place, the rest of the patch is generated with:
 spatch --sp-file scripts/coccinelle/qobject.cocci \
        --macro-file scripts/cocci-macro-file.h --dir . --in-place

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20170624181008.25497-3-eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
diff --git a/monitor.c b/monitor.c
index d8ac20f..61b7982 100644
--- a/monitor.c
+++ b/monitor.c
@@ -3842,7 +3842,7 @@
 
     req_json = qobject_to_json(req);
     trace_handle_qmp_command(mon, qstring_get_str(req_json));
-    qobject_decref(QOBJECT(req_json));
+    QDECREF(req_json);
 
     rsp = qmp_dispatch(cur_mon->qmp.commands, req);