libvirt and virtualization tags are synonyms: https://askubuntu.com/tags/virtualization/synonyms. So, as Tachyons said in one comment, no need to use it separatly.
See https://askubuntu.com/privileges/suggest-tag-synonyms:
What happens when a question is asked using a synonym?
Any tags that match active synonyms will be automatically and silently changed from their original as-entered form to the tag that the synonym points to.
This also increments a synonym counter, which you can view at /tags/synonyms to evaluate the effectiveness (or ineffectiveness) of the synonyms that are in place.
