x86/mm: split p2m ioreq server pages special handling into helper

x86/mm: split p2m ioreq server pages special handling into helper

So that it can be shared by both ept, npt and shadow code, instead of
duplicating it.

No change in functionality intended.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>