Of the two functions portio_list_del and portio_list_destroy,
the latter is just freeing a memory area.  However, portio_list_del
is the logical equivalent of memory_region_del_subregion so
destruction of memory regions does not belong there.

Actually, neither of these APIs are in use; portio is mostly used by
ISA devices or VGAs, and neither of these is currently hot-unpluggable.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>