Generate correct xfree86 keymaps when evdev is used

Use a mapping table to map xfree86 keycodes to evdev keycodes and obtain
consistent keysyms for keymaps. This way, it is now possible to generate
correct keymaps when evdev driver is used on host.
Merge pull request #180 from mgorny/evdev-genkeys

Generate correct xfree86 keymaps when evdev is used