Document instancetype.kubevirt.io labels used to decorate objects

[lyarwood]

What happened:

$subject, something along the lines of https://github.com/kubevirt/common-templates/blob/master/templates/LABELS.md

What you expected to happen:
A clear and concise description of what you expected to happen.

How to reproduce it (as minimally and precisely as possible):
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Additional context:
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Environment:

• KubeVirt version (use virtctl version): N/A
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