Unbuffered QFile buffers when opened using FILE* handle

When a QFile is opened using QFile::open(FILE* stream, ... | QIODevice::Unbuffered), there is still some buffering happening in the stlib fwrite(..., stream).

When the QFile is used directly, this can be somewhat mitigated by calling QFile::flush, but when it is passed on as a QIODevice this is only possible by q_object_cast'ing it to QFile.