ia64/xen: xencomm conversion functions for hypercalls

On ia64/xen, pointer arguments for hypercall is passed
by pseudo physical address(guest physical address.)
So such hypercalls needs address conversion functions.
This patch implements concrete conversion functions for
such hypercalls.

Signed-off-by: Akio Takebe <takebe_akio@jp.fujitsu.com>
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>