pyloxi: keep track of offset/length ourselves in OFReader

For skip_align to work we need to know the offset from the beginning of the
message, not from the beginning of our OFReader (which might have already been
misaligned).
pyloxi: deal with length fields inside unpack

This will make list parsing much simpler.
pyloxi: remove special case for reading OXM lists

This is now handled automatically by match_v3.unpack.
pyloxi: generic list deserialization

Since objects now handle their own type and length fields they can all use the
same generic list code. The oftype functions now generate list pack, unpack,
and init code on demand so nothing needs to be changed in pyloxi to use a new
list type.
pyloxi: move pack_list to generic_util

For consistency with unpack_list.