x86/mm: Don't trigger unnecessary shadow scans on p2m entry update

x86/mm: Don't trigger unnecessary shadow scans on p2m entry update

When updating a p2m entry, the hypervisor scans all shadow pte's to find
mappings of that gfn and tear them down. This is avoided if the page count
reveals that there are no additional mappings. The current test ignores the
PGC_allocated flag and its effect on the page count.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Signed-off-by: Adin Scannell <adin@scannell.ca>
Signed-off-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>