x86/mm: Fix paging_load

x86/mm: Fix paging_load

When restoring a p2m entry in the paging_load path, we were not updating the
m2p entry correctly.

Also take advantage of this to act on an old suggestion: once done with the
load, promote the p2m entry to the final guest accessible type. This simplifies
logic.

Tested to work with xenpaging.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>