Use Cloudstack and choose the VM server you like (Score 1)

Why not consider Cloudstack (http://cloudstack.apache.org/docs/en-US/index.html) ? It works with varied virtualization back-ends - Xen, KVM, VMWare, Oracle VM. Has a simple browser-based interface for management as well as for users. It has LDAP integration.And of course a rich API that can even work with AWS. Start with a simple standalone system using the Runbook (http://people.apache.org/~ke4qqq/runbook/). Then expand from there to more VM server nodes.