* Allow more complex network topologies in distributed tests. Each

  machine can now declare an option `virtualisation.vlans' that causes
  it to have network interfaces connected to each listed virtual
  network.  For instance,

    virtualisation.vlans = [ 1 2 ];

  causes the machine to have two interfaces (in addition to eth0, used
  by the test driver to control the machine): eth1 connected to
  network 1 with IP address 192.168.1.<i>, and eth2 connected to
  network 2 with address 192.168.2.<i> (where <i> is the index of the
  machine in the `nodes' attribute set).  On the other hand,
  
    virtualisation.vlans = [ 2 ];

  causes the machine to only have an eth1 connected to network 2 with
  address 192.168.2.<i>.  So each virtual network <n> is assigned the
  IP range 192.168.<n>.0/24.

  Each virtual network is implemented using a separate multicast
  address on the host, so guests really cannot talk to networks to
  which they are not connected.

* Added a simple NAT test to demonstrate this.

* Added an option `virtualisation.qemu.options' to specify QEMU
  command-line options.  Used to factor out some commonality between
  the test driver script and the interactive test script.

svn path=/nixos/trunk/; revision=21928