This patch adds a channel event callback to the spice core interface.
This new callback will be called for three events:
(1) A new connection has been established.
(2) The channel is ready (i.e. authentication is done,
link message verification passed all tests, channel
is ready to use).
(3) Channel was disconnected.
Qemu will use this to send notifications to the management app.
This is the direction I wanna take with all interfaces: Clearly
separate interface (aka version information and function pointers)
and state information. SpiceKbdInterface defines the interface,
SpiceKbdInstance maintains per-instance state information. Keyboard
hasn't much beside a pointer to SpiceKbdInterface, for other
interfaces this very likely will be different.
