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>