ui/vc: split off the VC part from console.c

Move common declarations to console-priv.h, and add a new unit
console-vc.c which will handle VC/chardev rendering, when pixman is
available.

(if necessary, the move could be done chunk by chunks)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>