Serializer: fix endianness on magic; rewind before load.

* QDataStream swaps bytes on little-endian machines so
reinterpret_cast onto a quint32 just can't work.

* Create another QDataStream so that KCalCore can start with
deserializing the header. If we just pass along "input",
it is positioned after the header -- and can't go much further
anyway, it points to a buffer only as big as the header (!)