lib/packet/packet_base: fix signature of PacketBase.{parser, serialize}

All subclasses define them as
parser(cls, buf) and serialize(self, payload, prev)
Not parser(cls), serialize(self).

pylint also complains like
> W: 50,4:ipv4.parser: Arguments number differs from overridden method
> W: 65,4:ipv4.serialize: Arguments number differs from overridden method

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>