linux/x86: Adjust and fix page table handling.

linux/x86: Adjust and fix page table handling.

Ensure that all and only those page table entries that have their
present bit set undergo p2m/m2p translation in all relevant places.
This should fix migration issues with _PAGE_PROTNONE pages which
previously could retain MFNs in PTEs while having the present bit
clear (and thus were not getting (un)canonicalized during save/
restore).

Many thanks to Keir Fraser for his analysis, suggestions, and
corrections.

Signed-off-by: Jan Beulich <jbeulich@novell.com>