ia64/xen: define helper functions for xen hypercalls.

introduce helper functions for xen hypercalls which
traps to hypervisor.

Signed-off-by: Yaozu (Eddie) Dong <eddie.dong@intel.com>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Tony Luck <tony.luck@intel.com>