ia64/pv_ops/xen: define xen pv_cpu_ops.

define xen pv_cpu_ops which implementes xen paravirtualized
privileged instructions.

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>