Refactor kvm lookupIPFromStatusFile

[josedonizetti]

There are a few problems with the kvm lookupIPFromStatusFile.

1 - when the IP is found, it keeps executing the for instead of returning directly.
2 - it ignores the json.Unmarshal error due to a 'possible' empty file

This PR refactor the code to:

1 - return as soon as the IP match was found
2 - not ignore the JSON error, in case of an empty file it returns blank, but if the JSON itself is invalid we report the error because it means a possible error on the KVM installation of the user, which worths notifying.
3 - add tests to this logic

For the tests, I've used the same convention I saw for the hyperkit tests.

[k8s-ci-robot]

Hi @josedonizetti. Thanks for your PR.

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: josedonizetti
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[minikube-bot]

Can one of the admins verify this patch?

[tstromberg]

Looks great!

[tstromberg]

@minikube-bot OK to test

[tstromberg]

@minikube-bot OK to test