Right now we always assume pointers are stored as SPICE_ADDRESS,
i.e. 64bit, independent on the size sent on the network.
This is required for 64bit architectures of course, but slightly overkill
on 32bit architectures, so needs fixing when all SPICE_ADDRESS elements
can be made internal.
We move all message structs from spice-protocol to spice as
we want to be able to change these as needed internally. The
on-network format is no longer defined by these structures anyway,
but rather by the spice protocol description.
