ia64/xen: implement the arch specific part of xencomm.

On ia64/xen, pointer argument for the hypercall is passed
by pseudo physical address (guest physical address.)
So it is necessary to convert virtual address into pseudo physical
address right before issuing hypercall. The frame work is called
xencomm. This patch implements arch specific part.

Signed-off-by: Alex Williamson <alex.williamson@hp.com>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Cc: Akio Takebe <takebe_akio@jp.fujitsu.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>