This mimics what we already have for cryptsetup services: the slice they
are placed in (they have their own slice since that's what we do by
default for instantiated services) shouldn't conflict with
shutdown.target, so that veritysetup services can stay around until the
very end (which is what we want for the root and usr verity volumes).
It's literally just a copy of the same unit we already have for
cryptsetup, just with an updated description string.
