For every single ? in your comment, the answer is "it's done at the hypervisor level". A unikernel is basically a single process, so privilege separation, users, groups etc are irrelevant. Drivers are much, much simpler (and I would argue, easier to audit) because they mostly only need to access virtualized, hypervisor-provided HAL APIs. Filesystem, DB, LDAP would be external - possibly provided by another VM on the same physical hardware.