I have filed a bug report on Launchpad so hopefully this will get addressed. This is a bug specific to Ubuntu or at least Debian releases, was not a problem with Redhat.
According to the manual page, restarting libvirtd is not supposed to interrupt running virtual machines, but it does. Thus when an upgrade to libvirtd comes out on the host machines, if I apply it, it frequently restarts the guest machines which is unpopular with customers to put it mildly.
This is how to reproduce it, create kvm-qemu guests, with a dozen of them or so running on a box, on the host machine type systemctl restart libvirt-bin and on average about 3/4ths of the guest machines will reboot.