bhyve should block leases on drv purge
When purging (forcing them to release their holds) consumers of the vmm_drv interface which have attached to a given instance, bhyve should also go through the logic which blocks leases from being established through those existing vmm_hold references. Any lingering leases at that point (such as those used from the viona ring threads) will then be notified so they can drop their lease, cleaning out references to the to-be-destroyed vmm instance.
This was found while testing bits drafted for #14755. It was possible for bhyve consumer processes to get hung up on the way down, since the ring worker threads were not notified of the pending destruction.