Since with the current design there's one device per domain, and one

Since with the current design there's one device per domain, and one
domain per pagetable, there's no need for a backpointer to the domain
in the pagetable entry descriptor.  There can't be any other domain.
Also since there's no list, no list entry member is needed either.

This reduces early allocation to half of the previous size.  I think
it's possible to reduce it even further and not need a pagetable entry
descriptor at all, but I need to think about that a bit more.