x86/mm: handle foreign mappings in p2m_entry_modify

x86/mm: handle foreign mappings in p2m_entry_modify

So that the specific handling can be removed from
atomic_write_ept_entry and be shared with npt and shadow code.

This commit also removes the check that prevent non-ept PVH dom0 from
mapping foreign pages.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>