ioport: add destructor method to IORange

Previously all callers had a containing object with a destructor that
could be used to trigger cleanup of the IORange objects (typically
just freeing the containing object), but a forthcoming memory API
change doesn't fit this pattern.  Rather than setting up a new global
table, extend the ioport system to support destructors.

Signed-off-by: Avi Kivity <avi@redhat.com>