keymap: record multiple keysym -> keycode mappings

Sometimes the same keysym can be created using different key
combinations.  Record them all in the reverse keymap, not only
the first one.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20180222070513.8740-5-kraxel@redhat.com