Create carthage.libvirt plugin

[srak289]

This merge creates the carthage.libvirt plugin which operates the same as carthage/vm.py.
We create an import shim at carthage.vm for backwards-compatability that prints a warning.

[hartmans]

@srak289 Why do you import carthage.libvirt in carthage.init?
It seems undesirable to import the plugin but not to enable it.

[srak289]

@hartmans carthage/__init__ originally imported carthage.vm and setup the LibvirtDeployableFinder. I had originally updated the references to point to carthage.libvirt, but since libvirt is now a plugin the plugin's init handles that. I'll remove the import from the top-level.