xen: fix for xen guest with mem > 3.7G

PFN_PHYS() can truncate large addresses unless its passed a suitable
large type.  This is fixed more generally in the patch series
introducing phys_addr_t, but we need a short-term fix to solve a
Xen regression reported by Roberto De Ioris.

Reported-by: Roberto De Ioris <roberto@unbit.it>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>