public/io/netif.h: document a mechanism to advertise carrier state

public/io/netif.h: document a mechanism to advertise carrier state

This patch adds a specification for a 'carrier' node in xenstore to allow
a backend to notify a frontend of it's virtual carrier/link state. E.g.
a backend that is unable to forward packets from the guest because it is
not attached to a bridge may wish to advertise 'no carrier'.

While in the area also fix an erroneous backend path description.

NOTE: This is purely a documentation patch. No functional change.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Juergen Gross <jgross@suse.com>