Input: xen-kbdfront - add multi-touch support

Extend xen_kbdfront to provide multi-touch support to unprivileged domains.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
[dtor: factor out various sub-protocols - multitouch, single touch, keys]
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>