x0vncserver: keyEvent(): use scancodes if available

x0vncserver: keyEvent(): use scancodes if available

Use the client provided (using QEMU extendend key event) scancodes if
available instead of reverse mapping the keysym.

X11 unfortunately uses keyboard driver specific keycodes and provides
no direct way to query this, so guess based on the keyboard mapping.

Handle the two most likely keyboard drivers, the old xorgkbd and evdev.

Signed-off-by: Peter Korsgaard <peter.korsgaard@barco.com>