Fix broken QBluetoothServiceInfo::serviceClassUuids() call

It never returned a valid entry due to wrong QVariant conversion.
The patch changes the public header with a potential to break BC. A
formerly inline function was reimplemented and is no longer inline.
This should be safe though as older header versions can still use
the previous implementation.

Change-Id: If786a366e625a56810b8d4cc682b25d07f72f4e5
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>