VM builds: Use Linux 3.10

3.4 apparently gives corrupt 9pfs data on x86.  See e.g.

  http://hydra.nixos.org/build/5661036

where reading /nix/store/kfldnrrsq0lbbv13gjxdfb3vb1sbaz88-vm-run-stage2
gives garbage.  Strangely it doesn't happen here:

  http://hydra.nixos.org/build/5658978

Ah well.