x86/xen: fix arbitrary_virt_to_machine()

While I realize that the function isn't currently being used, I still
think an obvious mistake like this should be corrected.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Acked-by: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>