Bluetooth - device (LE) discovery for iOS

Add a QBluetoothDeviceDiscoveryAgent for iOS, implementation is based on
Core Bluetooth framework. Low Energy devices scan only.
- Ctor with address: set error as invalid bluetooth adapter error,
  Core Bluetooth framework does not provide access to the local adapter's address.
- Adjust a test - we do not have access to the 'local device' information on iOS
  (no address, no number of devices etc.) - skip the last test in tst_deviceDiscovery.

Change-Id: I49080d021c340016aebc548cc8ed758777c66397
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>