qapi: add support for mice with extra/side buttons

Adds "side" and "extra" values to enum InputButton. The naming was borrowed
from evdev since it is more descriptive than "button4" and "button5".

Signed-off-by: Fabian Lesniak <fabian@lesniak-it.de>
Message-id: 20161206190007.7539-2-fabian@lesniak-it.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>