When a message has been read from the network we now pass it into
the generated demarshaller for the channel. The demarshaller converts
the network data to in-memory structures that is passed on to the
spice internals.
Additionally it also:
* Converts endianness
* Validates sizes of message and any pointers in it
* Localizes offsets (converts them to pointers)
* Checks for zero offsets in messages where they are not supported
Some of this was previously done using custom code in the client, this
is now removed.
