tools/xenstore: add read connection state for live update

tools/xenstore: add read connection state for live update

Add the needed functions for reading connection state for live update.

As the connection is identified by a unique connection id in the state
records we need to add this to struct connection. Add a new function
to return the connection based on a connection id.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Acked-by: Wei Liu <wl@xen.org>