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>