Cause: Many external programs that are used by libvirt (e.g. qemu-kvm) produce useful error messages that were not passed on to the admin by libvirt, which instead would sometimes just report that a command had failed.
Consequence: Sometimes a problem whose cause/resolution would have been trivial to discover by looking at the error output of a command, were instead very difficult to diagnose.
Fix: If any external command run by libvirt exits with a failure, its stderr output will be captured and added to the system log as a libvirt error.
Result: Many problems are now easier to diagnose, because better information is available.