Reimplement QIODevice::reset() properly

Reimplement QIODevice::reset() properly

The reset() function is not const.

tests/benchmarks/network/access/qnetworkreply/tst_qnetworkreply.cpp:324:
warning: 'FixedSizeDataGenerator::reset' hides overloaded virtual
function [-Woverloaded-virtual]
    virtual bool reset() const{
                 ^
src/corelib/io/qiodevice.h:112: hidden overloaded virtual function
'QIODevice::reset' declared here
    virtual bool reset();
                 ^

Change-Id: I022d902147f9c8fb71bee8c3e21b963b2d7bec55
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>