There's a ton of info on running opnsense as a virtual host. Is there any built in ability to host KVM containers on top of opnsense? For the easiest configuration and network layout I think this would be a great setup.
Is there anything about your use of freebsd that would prevent running KVM?
it would not make sense to run Linux code (KVM) on a BSD system. On BSD you may be interested in behyve and jails.