QBluetoothHostInfo is useless

QBluetoothHostInfo is effectively useless, since the default constructed object is invalid, and to set it with any useful info (name/bluetooth address), you'd already need to know that information a priori.

The default constructed object should contain the name and address of the default system radio if one exists, and invalid otherwise. If multiple system radios exist, there should be a mechanism to make that known and return `QBluetoothHostInfo` objects for the others as well.