xen: Sync up with the canonical protocol definitions in Xen

This is the sync up with the canonical definitions of the input,
sound and display protocols in Xen.

Changes to kbdif:
1. Add missing string constants for {feature|request}-raw-pointer
   to align with the rest of the interface file.

2. 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

3. Move multi-touch device parameters to backend nodes: these are
    described as a part of frontend's XenBus configuration nodes
    while they belong to backend's configuration. Fix this by moving
    the parameters to the proper section.

Unique-id field:
1. Add unique-id XenBus entry for virtual input and display.

2. Change type of unique-id field to string for sndif to align with
display and input protocols.

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>