[libvirt] how to get started with libvirt & central access control

Quite new to libvirt (and kvm). I played with a few vm's with libvirt/kvm and vnc/virsh/virt-manager. I would now like to implement access control for my vm's (of any format: xen, kvm, etc) to a remote backend (mysql/ldap/other). Where does one begin? I would later want to do the same but in the context of a cluster of hosts (each running multiple vm's). Thanks in advance for any advice. /juan