Issue description

This test doesn't appear to have succeeded in the last few months:

https://hydra.nixos.org/job/nixos/trunk-combined/nixos.tests.blivet.x86_64-linux

Steps to reproduce

$ nix-build nixos/release-combined.nix -A nixos.tests.blivet

The test hangs on current master and 17.09.

/cc @aszlig