xen: add return value to set_phys_to_machine()

set_phys_to_machine() can return false on failure, which means a memory
allocation failure for the p2m structure.  It can only fail if setting
the mfn for a pfn in previously unused address space.  It is guaranteed
to succeed if you're setting a mapping to INVALID_P2M_ENTRY or updating
the mfn for an existing pfn.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>