nixos/test-driver: stop blackholing `vde_switch` stderr

The code seems to have been ported from the old Perl code. It worked, but
had some smells:

1. It swallowed all `stderr` into a pipe we never read. In addition to
   making things difficult to debug, it could potentially result in
   `vde_switch` hanging if it gets blocked trying to write to a pipe
   that's full. Solution: just let `vde_switch` inherit our `stderr`.
2. If `vde_switch` failed to start, we just logged a warning and
   continued on. Now we crash.
3. We allocated a PTY to communicate with `vde_switch`'s management
   REPL. This was unnecessary: non-TTY stdin/stdout works fine.
4. There was an old TODO here about not blocking forever trying to read
   `vde_switch`'s stdout. I've addressed that by using `select` with a
   timeout.

NOTE: this does change the API surface area of the `VLan` class a bit:
there's no longer a `fd` attribute that points at a PTY master. I did an
unscientific grep through `nixos/tests` and couldn't find references to
`vlan[0-9].fd` nor `vlans[...].fd`.

0 files changed, 0 insertions, 0 deletions