Input: xen-kbdfront - enable auto repeat for xen keyboard frontend driver

Long pressed key could not show right in XEN vncviewer after tigervnc
client changed the way how to send repeat keys, from "Down Up Down Up
..." to "Down Down ... Up". This will report autorepeat to input by
checking if same key being pressed, and let handler process it finally.

Signed-off-by: Liang Yan <lyan@suse.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>