SPICE native client support

Existing graphical consoles all use an HTML5 client through a
websocket proxy. The poor performance of such consoles makes them
innaproriate for anything but the simplest uses. To provide users with
a much improved graphical console experience, this spec proposes a new
console type, spice-native, that will allow users to connect to their
server's SPICE console withs a native client. The architectural
changes to support this will be made as generic as possible in order
to enable the eventual extension of native client support to VNC and
RDP.