Fix Variant conversion in same ComThread

[vezjakv]

Variant conversion gets blocked when calling from same ComThread.
For example, this happens in asynchronous COM event handling when
iterating through properties of received COM object in event handler:

ISWbemSink sink = ClassFactory.createSWbemSink();
sink.advise(ISWbemSinkEvents.class,new ISWbemSinkEvents() {
    public void onObjectReady(ISWbemObject object, ISWbemNamedValueSet asyncContext) {
        System.out.println("Received event: " + object.getObjectText_(0));
        for (Com4jObject oprop: object.properties_()) {
           ISWbemProperty prop = oprop.queryInterface(ISWbemProperty.class);
           System.out.println(prop.name() + ": " + prop.value());
        }
    }
});

The above code will block in iterator part when retrieving and converting Variants.
The solution is to do conversion in Variant's ComThread if the same as calling ComThread.