kvm tools, ui: Move VNC specific framebuffer code to ui/vnc.c

kvm tools, ui: Move VNC specific framebuffer code to ui/vnc.c

This patch makes use of 'struct framebuffer' and moves the VNC code to ui/vnc.c
in preparation for other framebuffer output targets.

Acked-by: Ingo Molnar <mingo@elte.hu>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: John Floren <john@jfloren.net>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>