Input: xen-kbdfront - fix multi-touch XenStore node's locations

kbdif protocol describes multi-touch device parameters as a
part of frontend's XenBus configuration nodes while they
belong to backend's configuration. Fix this by reading the
parameters as defined by the protocol.

Fixes: 49aac820 ("Input: xen-kbdfront - add multi-touch support")

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>