Fix kvm remove when domain is not defined

[josedonizetti]

This is the second part to fix for #4319

In case someone undefined the domain outside minikube, the deletion would fail, so we check if the domain is actually defined before callding undefined on it.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @josedonizetti. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[minikube-bot]

Can one of the admins verify this patch?

[tstromberg]

Thank you!

[tstromberg]

@minikube-bot OK to test