Hi, after few days of work, here is implementation of Libvirt provider for Vagrant. This is first 0.0.1 version, so it supports just basic actions - up and destroy, nothing more for now. Other actions can be added, just after implementing some better way to find out domains IP address, which is little tricky with Libvirt.
Provider uploads box image into libvirt storage pool and there are also COW images for domains created. Currently only KVM machines are created.
Provider is usable localy, or to control remote Libvirt hosts.
More info in github repo: https://github.com/pradels/vagrant-libvirt