xen/p2m: Mark INVALID_P2M_ENTRY the mfn_list past max_pfn.

In case the mfn_list does not have enough entries to fill
a p2m page we do not want the entries from max_pfn up to
the boundary to be filled with unknown values. Hence
set them to INVALID_P2M_ENTRY.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>