xen/displif: Add unique display connector identifier

xen/displif: Add unique display connector identifier

If frontend is configured to expose multiple connectors then backend may
require a way to uniquely identify concrete virtual connector within the
frontend. This is useful for use-cases where connector needs to be
matched to physical display connector.
Add XenBus "unique-id" node parameter, so this sort of use-cases can
be implemented.

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>