Input: xen-kbdfront - allow better run-time configuration

It is now only possible to control if multi-touch virtual device
is created or not (via the corresponding XenStore entries),
but keyboard and pointer devices are always created.
In some cases this is not desirable. For example, if virtual
keyboard device is exposed to Android then the latter won't
automatically show on-screen keyboard as it expects that a
physical keyboard device can be used for typing.

Utilize keyboard and pointer device XenStore feature fields to
configure which virtual devices are created:
 - set "feature-disable-keyboard" to 1 if no keyboard device
   needs to be created
 - set "feature-disable-pointer" to 1 if no pointer device
   needs to be created
Keep old behavior by default.

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