Indirect writes to phys_to_machine_mapping array through an inline

Indirect writes to phys_to_machine_mapping array through an inline
function defined in page.h.

For self-migration, I need the ability to trap writes to the p2m map
during migration, because I keep a dictionary mapping mfns to pfns,
which I use for remapping the page tables once I am on the other side.

Signed-off-by: Jacob Gorm Hansen <jacobg@diku.dk>