Use direct access to the read buffer

1. Use the auxiliary array to store the chunks of data at reading
on Windows.

The additional array is used for temporary storing the data chunks of
the each asynchronous reading operation. This data will be appended to
the class read buffer when asynchronous operation completed. It reduce
quantity of calls for reallocation of the input ring buffer because now
the reallocation with the actual data is carried out once in the
completeAsyncRead() method.

2. Delete the bytesAvailable() and readFromBuffer() methods from
the QSerialPortPrivate.

These methods are not necessary any more, because now the size and content
of the read buffer always has actual value. Now all dependent methods
can access to the read buffer directly without the intermediaries,
like bytesAvailable() and readFromBuffer().

Tested the build with Qt4 and then Qt5 for:

* Windows 7/8, MinGW
* Windows CE Pocket PC, VS2005
* Android platforms ARM and x86
* Archlinux 64-bit

Tested the work with the terminal, the creader(a)sync, the cwriter(a)sync
examples with Qt4 and then Qt5 for:

* Windows 7/8 with the virtual com0com, the physical on-board and the
USB PL2303 devices
* Archlinux 64-bit with the physical on-board and the USB PL2303 devices

Change-Id: I6871f8933588377646ffeccf1d2c21bb6caada24
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Laszlo Papp <lpapp@kde.org>