interface: fix nested QDBusArgument in QVariant not accessible from QML

The Metadata property from mpris2 Player could not be read from QML because the returned type was QVariant(QDBusArgument) this fixes that case. I think the same happens if a instead of calling getProperty we make a typedCall so that might need to be fixed too. This might not be the best solution someone should take a look into this.

```javascript
DBusInterface {
    id: mpris2
    service: "org.mpris.MediaPlayer2.spotify"
    iface: "org.mpris.MediaPlayer2.Player"
    path: "/org/mpris/MediaPlayer2"
    Component.onCompleted: {
        console.log("Property: " + getProperty('PlaybackStatus'))
        var metadata = getProperty('Metadata');
        for (var prop in metadata)
            console.log(prop, "=", metadata[prop])
    }
}
```