docs: Introduce xenstore paths for guest network address information

docs: Introduce xenstore paths for guest network address information

It is useful for a toolstack to be able to see the network addresses
in use by a domain for a particular vif in xenstore for display
purposes and, for example, so that a VNC session can be established
to the guest GUI.

This patch documents paths to allow a domain to advertise an interface
name, MAC (unicast and multicast) and IP (version 4 and 6) address
information.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Tim Deegan <tim@xen.org>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>