lxc: report veth device indexes to systemd
    
    Record the index of each host-side veth device created and report
    them to systemd, so they show up in machinectl status for the VM.
    
    lxc-shell(95449419f969d649d9962566ec42af7d)
         Since: Fri 2015-01-16 16:53:37 GMT; 3s ago
        Leader: 28085 (sh)
       Service: libvirt-lxc; class container
         Iface: vnet0
       Address: fe80::216:3eff:fe00:c317%124
            OS: Fedora 21 (Twenty One)
          Unit: machine-lxc\x2dshell.scope
                └─28085 /bin/sh