I have a question asked recently here:
which I am unable to tag with libvirt If I tag it, it disappears when the question is up on AU. What is going wrong here?
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.
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.