[tag:libvirt] and [tag:virtualization] tags are synonyms: https://askubuntu.com/tags/virtualization/synonyms. So, as [Tachyons](http://meta.askubuntu.com/users/39372/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](https://askubuntu.com/tags/synonyms) to evaluate the effectiveness (or ineffectiveness) of the synonyms that are in place.