CEC: Discover CEC devices whenever Solid reports a new device

Instead of polling, use the OS mechanisms to detect new devices and then
ask whenever there are CEC devices at a point in time.

While not entirely accurate, it's unclear what class CEC devices are but
it's still better than any polling as, either way, they should be
getting announced.

We also discover devices at startup for integrated devices, which I
understand should be the most common case anyway.