I've stopped using Virtualbox altogether because Oracle no longer submits
security reports in new updates (e.g. they will release a security update and
not say what the issue fixed was). For you though, it's kept offline so this
probably doesn't matter much anyway.
I've personally transitioned to using KVM with virt-manager because of the
above stated info.
Lastly, it is possible to convert virtualbox .vdi to KVM .qcow2 virtual
images. http://blog.bodhizazen.net/linux/convert-virtualbox-vdi-to-kvm-qcow/
The blog post I linked was from 2009 so some process in converting might have
changed but it is possible.
