Add function to enumerate possible/currently available devices

QCanBus::instance()->plugins() already gives a list of available plugins - they may not actually useable (e.g. due to missing hardware drivers).

There should be a possibility to enumerate the devices provided by a backend - in best case the currently available ones, second best the supported ones (e.g. peakcan supports usbbus1...usbbus8 but only usbbus1 is available)