xen: export get_phys_to_machine

-tip testing found the following xen-console symbols trouble:

  ERROR: "get_phys_to_machine" [drivers/video/xen-fbfront.ko] undefined!
  ERROR: "get_phys_to_machine" [drivers/net/xen-netfront.ko] undefined!
  ERROR: "get_phys_to_machine" [drivers/input/xen-kbdfront.ko] undefined!

with:

  http://redhat.com/~mingo/misc/config-Mon_Jun__2_12_25_13_CEST_2008.bad