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.