Problem
After an Update the bridged interface got disconnected. No virtual machine (VM) has any network connection.
$ brctl show
bridge name bridge id STP enabled interfaces
br0 8000.000f3d57bde7 no
Solution
Re-attach the bridge interface to the correct main interfaces.
$ brctl addif br0 vnet0
$ brctl show
bridge name bridge id STP enabled interfaces
br0 8000.000f3d57bde7 no vnet0