Move all platform-specific declarations into one QSPPrivate

This is near to the internal Qt PIMPL style. Also it will allow
to use QIODevicePrivate is more smoothly in next patches.

Tested on Windows 8, Linux, OSX 10.9 with on-board serial port.

Change-Id: Ieef4b05d0bf11cf2da6420e58629bea1818108f9
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>