[libvirt] Dummy tag in xml desc?

Hello Everybody! I'd like to find a way to pass information between a machine & a virtual machine managed by libvirt (like we can with VirtualBox or VMware thanks to guest tools for instance ). I thought that I would have been able to write a comment section or something like that within the xml description of the right domain, but every new comments or xml tags are erased once I save them... Maybe somebody's got an idea to help me :D... JMi