monitor: Reduce handle_qmp_command() tracing overhead

We are malloc'ing a QString and spending CPU cycles on converting a
QObject to string, just for the sake of sticking the string in the trace
message.  Wasted when we aren't tracing.  Avoid that.

[Commit message and description suggested by Markus Armbruster to
provide more detail about the rationale for this patch.

Use trace_event_get_state_backends() instead of trace_event_get_state()
to honor DTrace/UST backend dstates.
--Stefan]

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20170725143923.11241-1-den@openvz.org
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Lluís Vilanova <vilanova@ac.upc.edu>
CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
CC: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

monitor.c

@@ -3823,7 +3823,6 @@ static void handle_qmp_command(JSONMessageParser *parser, GQueue *tokens)
     QDict *qdict = NULL;
     Monitor *mon = cur_mon;
     Error *err = NULL;
-    QString *req_json;
 
     req = json_parser_parse_err(tokens, NULL, &err);
     if (!req && !err) {
@@ -3841,9 +3840,11 @@ static void handle_qmp_command(JSONMessageParser *parser, GQueue *tokens)
         qdict_del(qdict, "id");
     } /* else will fail qmp_dispatch() */
 
-    req_json = qobject_to_json(req);
-    trace_handle_qmp_command(mon, qstring_get_str(req_json));
-    QDECREF(req_json);
+    if (trace_event_get_state_backends(TRACE_HANDLE_QMP_COMMAND)) {
+        QString *req_json = qobject_to_json(req);
+        trace_handle_qmp_command(mon, qstring_get_str(req_json));
+        QDECREF(req_json);
+    }
 
     rsp = qmp_dispatch(cur_mon->qmp.commands, req);