x86/p2m: change_entry_type_* hooks are HVM-only

x86/p2m: change_entry_type_* hooks are HVM-only

Exclude functions using them from !HVM builds, thus making it possible
to exclude the hooks as well. Also cover the already unused
memory_type_changed hook while inserting the #ifdef in the struct.

While no respective check was there, I can't see how
XEN_DOMCTL_set_broken_page_p2m could have been meant to work for PV the
way it is implemented. Restrict this operation to acting on HVM guests.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>