Until now, the unifying plugin was a 'special snowflake' and did things in very different ways to the other plugins. Refactor each object to derive from either FuUsbDevice or FuUdevDevice, and then remove LuDevice and the LuContext layers.
This shouldn't have an effect now, but is required when the device list starts handling the replug, and it seems logically correct.
A small optimization, which avoids us querying all the USB plugins to find a match each time.
