Testing-farm reports problem like a machine that fails to boot as `internal/guest` as fail subresult, but this causes the overall test result to be error.

```
<check name="internal/guest" result="fail" event="after-test">
```
In restraint a problem like this would also be reported as error. To make things consistent we should report this as an error subresult as well.

Jira: CKI-6483