Disable predictable interface names in tests

Apparently systemd is now smart enough to figure out predictable names
for QEMU network interfaces. But since our tests expect them to be
named eth0/eth1..., this is not desirable at the moment.

http://hydra.nixos.org/build/10418789

nixos/modules/virtualisation/qemu-vm.nix

@@ -402,6 +402,8 @@ in
     # Speed up booting by not waiting for ARP.
     networking.dhcpcd.extraConfig = "noarp";
 
+    networking.usePredictableInterfaceNames = false;
+
     system.requiredKernelConfig = with config.lib.kernelConfig;
       [ (isEnabled "VIRTIO_BLK")
         (isEnabled "VIRTIO_PCI")