kvm tools, ui: Add some missing key codes

Add some missing keys to the SDL ui. This needed a bit of rework of
keyboard event handling since some of the new codes are multi-byte,
so I had to introduce a struct that keeps that information alongside
the code itself. Peculiar cases like PAUSE or PRNT SCRN are special
cased, however they aren't wired yet since I couldn't test them (I think
input grabbing is required to make them work).

Acked-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Alessandro Guido <ag@alessandroguido.name>
Signed-off-by: Pekka Enberg <penberg@kernel.org>