This is a zsh autocompletion file for libvirt's virsh command.
Working for libvirt >= version 0.9, tested with libvirt 1.2.12 on Ubuntu
cp _virsh /usr/share/zsh/functions/Completion/Linux
Actually, any path in your $fpath would work, so if you like that better, use /usr/local/share/zsh/site-functions for example.
After copying, restart your shell.