ia64/xen: define helper functions for xen related address conversion.

Xen needs some address conversions between pseudo physical
address (guest phsyical address), guest machine address
(real machine address) and dma address.
Define helper functions for those address conversion.

Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Tony Luck <tony.luck@intel.com>