Try to have a common base dispose() method for channels. For now, it
just free the caps.
Make use of it in snd_worker, and in sync_write() - sync_write() is
going to have default caps later on.
https://bugs.freedesktop.org/show_bug.cgi?id=34795
This is stylish change again. We are talking about a RedStream object,
so let's just name the variable "stream" everywhere, to avoid
confusion with a non existent RedPeer object.
https://bugs.freedesktop.org/show_bug.cgi?id=34795
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.
