This commit updates how Vagrant manages disk state with the virtualbox provider. Instead of using the raw structure from `list_hdds` for all_disks, it instead uses that data that now lives inside the controllers attachment structure.
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.