If you're like me, you run libvirt on a headless server and look at VM consoles with virt-viewer. You also probably see the following warnings:

Gtk-Message: 14:08:40.348: Failed to load module "canberra-gtk-module"
Gtk-Message: 14:08:40.348: Failed to load module "pk-gtk-module"
Gtk-Message: 14:08:40.477: Failed to load module "canberra-gtk-module"
Gtk-Message: 14:08:40.477: Failed to load module "pk-gtk-module"

The solution is as follows

yum install PackageKit-gtk3-module libcanberra-gtk3