[libvirt] Bite sized task: #pragma once

A heads up that I added a trivial bite sized task to convert header files to use #pragma once https://wiki.libvirt.org/page/BiteSizedTasks#Switch_headers_to_use_.23pragma... Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|