TODO
====
-* Golden image / snapshotting / expandable VMs.
-* Support more VBoxManage commands.
-* Run vbox upgrade if available on all available instances.
-* Iteration over a set of VMs.
-* Support for upgrading VMs.
-* Vagrant integration.
-* Set window title using xdotool.
-* Check for `/usr/bin/VBoxClient --clipboard` inside the guest.
-* [host's resolver as a DNS proxy in NAT mode](https://www.virtualbox.org/manual/ch09.html#nat_host_resolver_proxy):
- `VBoxManage modifyvm "VM name" --natdnshostresolver1 on`.
+* Vagrant integration with upgrade procedure.