Enable creating an image model from an existing image

[tbatchelli]

e.g. you already have an image working, and you want to reuse it as an image model.

This feature would require for the user to supply the meta data for the image and the image name, and vmfest would prepare the image to become a model and then install it as a model locally.

To become a model we need to: 1- remove any persistent net rules 2- Clean log files, caches, etc 3- Ensure at least 2 network interfaces are active (optional) 4- Ensure guest additions are installed (optional)

3- and 4- should be options of the function to create such models. This function could be tentatively called "remodel"

Another use case is importing Vagrant .box VMs. These would require some work to get NAT+HostOnly to work, because most linuxen by default only activate one network interface.