Split `vm/` directory (i.e. Vagrant boxes) in separate git repo

The new home of the Vagrant boxes is
https://github.com/lucafavatella/vagrant-boxes.

The main command used is:
```
$ git subtree split --prefix=vm --branch=temporary-split-branch
```
Source: https://stackoverflow.com/questions/920165/howto-extract-a-git-subdirectory-and-make-a-submodule-out-of-it