minimega: not all errors are propagated to log files

[djfritz]

Describe your environment

1. minimega version
   2.6

Describe the bug
Some errors are not being propagated anywhere but stderr, even if stderr logging is disabled:

log stderr false
log file foo

# will log to foo
broken command

# will not log to foo (see #1376 as to why this is an error)
vm config net a
unable to determine valid QEMU hosts -- exec: "kvm": executable not found in $PATH

Expected behavior
The second error should log to the file, not to stderr.

[djfritz]

I'm guessing minicli/output.go needs to have Responses' String() and Error() functions reworked a bit. We want to push errors to the logging subsystem and non-error responses along the normal path, but it's not currently possible to separate those for later consumption by the cli back in minimega (String() joins them currently it looks like).

@jcrussell thoughts on changes here?