Support peer-to-peer networking of VM instances on a LAN

[bgilbert]

Some VMs (e.g. multiplayer games) might want to be able to network with other instances of the same VM. Allow a VM to register itself in mDNS and create an overlay network with other VMs (e.g. using VDE).

[bgilbert]

The overlay network implementation will have to contend with the fact that every VM on the network will have the same MAC address (assuming only one VM image is involved). We don't randomize MAC addresses at startup because that would confuse the guest OS.