Implement the initialization of QBluetoothLocalDevice

The instance of the local device is initialized via its bluetooth address.

* If the constructor is passed an empty address, the class will be
initialized using properties of first found local Bluetooth device in
the system.

* If the constructor is passed the specific address, it will
enumerated all local Bluetooth devices and compares the Bluetooth
address of each device. In case none of the addresses match,
QBluetoothLocalDevice::isValid() returns false.

Change-Id: Ib88df0403aa5d227ec6b75a4afcbf2f8675f05a8
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>