Revert QDateTime serialisation to pre-Qt 5 behaviour.

In Qt 5, I managed to break the guarantee that a deserialised local
datetime is the same time of day (potentially different UTC time),
regardless of which timezone it was serialised in. This happened after
I fixed QTBUG-4057 with If650e7960dca7b6ab44b8233410a6369c41df73a,
which serialised datetimes as UTC.

This patch reverts QDateTime serialisation to pre-Qt 5 behaviour to
restore the guarantee and consequently re-opens QTBUG-4057.

Change-Id: Iea877f7ed886f530b928067789b53534e89fe8cb
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>