xen/x86: remove foreign mappings from the p2m on teardown

xen/x86: remove foreign mappings from the p2m on teardown

Iterate over the p2m up to the maximum recorded gfn and remove any foreign
mappings, in order to drop the underlying page references and thus don't keep
extra page references if a domain is destroyed while still having foreign
mappings on it's p2m.

The logic is similar to the one used on Arm.

Note that foreign mappings cannot be created by guests that have altp2m or
nested HVM enabled, as p2ms different than the host one are not currently
scrubbed when destroyed in order to drop references to any foreign maps.

It's unclear whether the right solution is to take an extra reference when
foreign maps are added to p2ms different than the host one, or just rely on the
host p2m already having a reference.  The mapping being removed from the host
p2m should cause it to be dropped on all domain p2ms.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>