The libvirt-guests script executed operations on guests serially.
On machines with lots of guests the shutdown procedure took long as guests were waiting for shutdown of others. The shutdown procedure was inefficient as guests didn't use up all resources available.
The libvirt-guests init script was tweaked to enable parallel operation on domains allowing to shorten the time of shutdown of the host.
The guests start and shutdown in parallel and utilize the host system's resources more efficiently. The shutdown time of the host will decrease in most cases.