We're going to be calling some methods from AllocImpl directly in
Registration::new() - which means that the rust compiler needs to know that
our associated AllocImpl trait uses us as a Driver trait. Otherwise, it'll
complain about a missing trait bound.

The simplest way to do this is to just add this as a trait bound for the
associated Driver::Object trait.

Signed-off-by: Lyude Paul <lyude@redhat.com>