x86/PoD: shorten certain operations on higher order ranges

x86/PoD: shorten certain operations on higher order ranges

Now that p2m->get_entry() always returns a valid order, utilize this
to accelerate some of the operations in PoD code. (There are two uses
of p2m->get_entry() left which don't easily lend themselves to this
optimization.)

Also adjust a few types as needed and remove stale comments from
p2m_pod_cache_add() (to avoid duplicating them yet another time).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>