docs: document that vfio is default for hostdev networks too

docs: document that vfio is default for hostdev networks too

When the default was changed from kvm to vfio, the documentation for
hostdev and interface was changed, but the documentation in <network>
was forgotten.

Also document when the default was changed from "always kvm" to "vfio
if available, else kvm" (1.0.5).