Use QIODevicePrivate for PIMPL

It is reasonable to get rid of excess D/Q pointers and to use their
equivalents from QIODevicePrivate. Also in the future it will allow
to use the internal read buffer of QIODevicePrivate.

Tested on Windows 8 and Linux with the on-board and com0com serial
ports.

Change-Id: I9d0923c6f9adc5981dc49dacea163b286620ed5c
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>