ia64/xen: implement arch specific part of xen grant table.

Xen implements grant tables which is for sharing pages with
guest domains.
This patch implements arch specific part of grant table initialization.
and xen_alloc_vm_area()/xen_free_vm_area() which are helper functions
for xen grant table.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Tony Luck <tony.luck@intel.com>