xen/kbdif: Add features to disable keyboard and pointer

xen/kbdif: Add features to disable keyboard and pointer

It is now not fully possible to control if and which virtual devices
are created by the frontend, e.g. keyboard and pointer devices
are always created and multi-touch device is created if the
backend advertises multi-touch support. In some cases this
behavior is not desirable and better control over the frontend's
configuration is required.

Add new XenStore feature fields, so it is possible to individually
control set of exposed virtual devices for each guest OS:
 - 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>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Release-acked-by: Juergen Gross <jgross@suse.com>