Comment 18 for bug 199533

another debugging thing to try...

I think that maybe the vif interfaces are missing or having trouble coming up

Try putting the following in your /etc/network/interfaces

auto br0
iface br0 inet static
  address 192.168.133.7
  netmask 255.255.255.0
  broadcast 192.168.133.0
  bridge_ports eth0
  bridge_fd 9
  bridge_hello 2
  bridge_maxage 12
  bridge_stp off

Then run:
/etc/init.d/networking restart

Then in the guest try changing the vif line to add

vif=['bridge=br0']

Then try to start the guest.