Make it possible to specify name of imported box

[varac]

I'm using libvirt as provider. Unfortunatly, there are not much boxes around for this provider. so i need to manually download a box via wget, and mutate it. but then, the imported box is named after the filename, and upstream Vagrantfiles often use the hashicorp atlas notation with a slash in the middle, i.e. 'ubuntu/wily'.
There is no way to name a filename like this. So in order to not change the box name in the Vagrantfile, i need to repackage it and import it again giving the box a name with:

vagrant box add --name ubuntu/wily64 ubuntu_wily64.libvirt.box

I'd like to be able to give the mutated box a name like this:

vagrant mutate --name ubuntu/wily64 --input-provider virtualbox ubuntu_wily64_virtualbox.box libvirt

[sciurus]

Sounds like a good idea; I'd be happy to accept a pull request adding this.