The current conversion code for QIODevice.readData(), readLineData(), etc. is
here (PySide/QtCore/typesystem_core.xml):

                    %out = PyString_GET_SIZE((PyObject*)%PYARG_0);
                    qstrncpy(%1, PyString_AS_STRING((PyObject*)%PYARG_0), %out
+ 

This conversion doesn't work correctly if return data have '\0' in the middle
of them.

This code must use qmemcpy instead of qstrncpy, because QIODevice.readData()
must be able to return arbitrary binary data which may have some '\0'.