xen: introduce the header file for the Xen 9pfs transport protocol

It uses the new ring.h macros to declare rings and interfaces.

CC: konrad.wilk@oracle.com
CC: boris.ostrovsky@oracle.com
CC: jgross@suse.com
CC: groug@kaod.org

Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
Acked-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>