QBluetoothServiceDiscoveryAgent::InvalidBluetoothAdapter was introduced
by Qt 5.3 but never added to the QML BluetoothDiscoveryModel. This patch
fixes the problem.

This was noticed due to a crash on Android emulator.

Change-Id: I652576929659ca370216133154e36158e8425711
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>