This means the member is not sent on the network at all.
Instead its initialized to the attribute argument when demarshalled.
This is useful for backwards compatibility support.
This is required because we don't want to free messages that just
refer to the unparsed message (like SpiceMsgData).
Also, in the future we might need it for more complex demarshalling.
The "spice.proto" file describes in detail the networking prototcol
that spice uses and spice_codegen.py can parse this and generate
demarshallers for such network messages.
