Change Glib::Variant<uint64_t> to Glib::Variant<guint64>.

This is needed on OSX/x86_64 where one is a typedef to unsigned long and the
other to unsigned long long. Both are the same size, but they are still
considered different types for the purposes of template instantiation, so the
specialisations which are defined for Glib::Variant<guint64> are not found if
Glib::Variant<uint64_t> is used.