xen/evtchn: Fix name of Xen event-channel device

The Xen event-channel device is named evtchn in the kernel but always
used as /dev/xen/evtchn in userspace. This patch fixes the name.

Signed-off-by: Bastian Blank <waldi@debian.org>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>