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.
VDInterface has been renamed to SpiceBaseInterface. Dropped base_version
element, shlib versioning should be used instead. Dropped id element,
it is passed to spice_server_add_interface() instead. Now
SpiceBaseInterface has static information only, multiple interface
instances can share it.
Added SpiceBaseInstance struct for maintaining per-instance state
information. Adapted spice_server_{add,remove}_interface() functions
to the new world.
