x86/PVH: de-duplicate mappings for first Mb of Dom0 memory

x86/PVH: de-duplicate mappings for first Mb of Dom0 memory

One of the changes comprising the fixes for XSA-378 disallows replacing
MMIO mappings by code paths not intended for this purpose. This means we
need to be more careful about the mappings put in place in this range -
mappings should be created exactly once:
- iommu_hwdom_init() comes first; it should avoid the first Mb,
- pvh_populate_p2m() should insert identity mappings only into ranges
  not populated as RAM,
- pvh_setup_acpi() should again avoid the first Mb, which was already
  dealt with at that point.

Fixes: 753cb68e6530 ("x86/p2m: guard (in particular) identity mapping entries")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>