QEMU/KVM - What does xmlns:qemu do? I want to use QEMU command line options in a VM's XML configuration. This requires me to add <domain type='kvm' xmlns:qemu=' in the beginning like this. I'm wondering what this option does, since I am wary of adding it, considering it seems to contain a URL to an internet resource, and it is a HTTP link. Is this a potential security risk? What does this download, if anything? Could it lead to compromise of the VM or host, if maliciously modified by a man-in-the-middle attack?

That's an XML namespace definition, and the given URI is only used as an identifier; it doesn't cause anything to be downloaded (in fact if you try to use the URI here as a URL you'll see there's nothing there). The definition allows XML parsers to correctly validate the configuration file's syntax.