vm-configurator: Add and use metadata.libosinfo
Similarly to what has been done in virt-manager (commit bad5eabd9), let's have a cross-app schema for tracking libosinfo OS ID in the domain XML, as done in: <libosinfo:libosinfo xmlns:libosinfo="http://libosinfo.org/xmlns/libvirt/domain/1.0"> <libosinfo:os id="http://fedoraproject.org/fedora/17"/> </libosinfo:libosinfo>