Virtual Monitor: always provide dbus path

As mentioned in !670 (comment 945441), the DBus path should always be provided.

The plugin should always be loaded, as:

• we can provide a virtual display over VNC, even if the device isn't capable to use virtual displays itself (krfb-virtualmonitor)
• we would do the capabilities check regardless, if !670 (merged) gets merged
• hiding the DBus path doesn't trigger the PluginChecker.qml to think the plugin is unavailable, it just doesn't work
• -> is this a bug or intended behaviour?

I separated this MR, in case we further need to discuss it. !670 (merged) should be good to go, if you approve it.