vmd(8): allow locking a randomly assigned lladdr

vmd(8): allow locking a randomly assigned lladdr

Provide a default value of a zero'd mac address so a user can still
specify the interface should be "locked" (only transmitting ethernet
packets with a matching source address). vmd will assign a random
address at vm launch.

As an example, this is now valid:

  vm "name" {
    interface {
      locked lladdr
    }
    ...
  }

From Martin Vahlensieck
ok claudio@