libxl: introduce new backend type VINPUT

libxl: introduce new backend type VINPUT

There are two kind of VKBD devices: with QEMU backend and user space PV
backend. In current implementation they can't be distinguished as both use
VKBD backend type. As result, user space PV KBD backend is started and
stopped as QEMU backend. This commit adds new device kind VINPUT to be
used as backend type for user space PV KBD backend.

Signed-off-by: Oleksandr Grytsov <oleksandr_grytsov@epam.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>