Input: xen-kbdfront - use string constants from PV protocol

Xen input para-virtual protocol defines string constants
used by both back and frontend. Use those instead of
explicit strings in the frontend driver.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>