ia64/xen: introduce definitions necessary for ia64/xen hypercalls.

import arch/ia64/include/asm/xen/interface.h to introduce
definitions necessary for ia64/xen hypercalls.
They are basic structures to communicate with xen hypervisor and
will be used later.

Cc: Robin Holt <holt@sgi.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Tony Luck <tony.luck@intel.com>