Comment 21 for bug 1667113

I had been facing the same problem along with one more user account named "No value has been set".
For "Libvirt Qemu" tried purging "qemu-kvm" and it's configuration files. But still user account was existed. I had been trying for the solution and found one that you can also try. (I'm using linux mint 18.2 Sonya)

Go to location /etc/ as root and find a file named "passwd" and open it. Now find the line
"libvirt-qemu:x:64055:132:Libvirt Qemu,,,:/var/lib/libvirt:/bin/false" (see the image I attached not to get confused, values may vary in your case) and clear it (don't clear any other line which is not related to "libvirt-qemu"). I found that line almost at the end of the list.

If you're having one more user account named "No value has been set" then clear the line below:
"<admin name>:x:1001:1002:No value has been set:/dev/null:/dev/null" (values may vary in your case)

And now I don't have these user accounts or any other issue after removing those values: "libvirt qemu" and "No value has been set"