phys_to_machine_mapping array is not an array of longs.

phys_to_machine_mapping array is not an array of longs.
Fix start-of-day code and FOREIGN_FRAME() macro in light
of this. Also fix pfn_to_mfn() to mask off the foreign-frame
marker bit. The few places where we rely on that bit we read
out of the array directly, rather than using the pfn_to_mfn()
accessor macro.

This fixes, among other things, booting domU on PAE with swiotlb
disabled.

Signed-off-by: Keir Fraser <keir@xensource.com>