This commit adds a new VirtualBox provider helper method to return a list of storage controllers so Vagrant can find a storage controller with the desired characteristics (type IDE or SATA). This still needs to get wired up to the disk cleanup method.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.