docs: update the xenstore migration stream documentation

docs: update the xenstore migration stream documentation

For live update of Xenstore some records defined in the migration
stream document need to be changed:

- Support of the read-only socket has been dropped from all Xenstore
  implementations, so ro-socket-fd in the global record can be removed.

- Some guests require the event channel to Xenstore to remain the same
  on Xenstore side, so Xenstore has to keep the event channel interface
  open across a live update. For this purpose an evtchn-fd needs to be
  added to the global record.

- With no read-only support the flags field in the connection record
  can be dropped.

- The evtchn field in the connection record needs to be switched to
  hold the port of the Xenstore side of the event channel.

- A flags field needs to be added to permission specifiers in order to
  be able to mark a permission as stale (XSA-322).

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>