[console] stderr output from exec duplicated into a separate fd (?)

Somehow stderr output gets duplicated into a separate FD, preventing its redirection into a file.

```
$ mp ls
Name State IPv4 Image
awesome-gemsbok Running 10.213.185.198 Ubuntu 18.04 LTS
$ ssh ubuntu@10.213.185.198 bash -c 'echo stderr >&2; echo stdout' 2>/dev/null
stdout

# compared to

$ multipass exec awesome-gemsbok -- bash -c 'echo stderr >&2; echo stdout' 2>/dev/null
stderr
stdout
$ multipass exec awesome-gemsbok -- bash -c 'echo stderr >&2; echo stdout' >/dev/null
stderr
```

Start by reproducing the issue with the shown multipass exec commands and shell redirections, then trace the console exec handling that forwards stdout and stderr. Done means stderr is suppressed by 2>/dev/null and stdout is suppressed by >/dev/null, matching the ssh behavior.

Written by the indexing model from the issue text.