Qemu is not better and if you need Windows guest, it's far worse. The work that steve does, is pretty reliable, but as you can see the OP came up with excuses to not use that either, With Qemu, I had no problems upgrading to Debian 1O either.but if the OP prefers to use the other, and does not like to use something better, so be it, I am sorry I suggested it.
Well it might seem irrelevant, but when someone else told me about Qemu, I also was still using VirtualBox, all my VM's were Virtual Box, and when Debian stopped including it in the repos, I did not know what to do, so when someone suggested trying qemu, I tried it, and it was such a big improvement, it was well worth the trouble of converting the other VM's to Qemu as well, (not all that much trouble), any way, I was really gratefull to learn about qemu, and have not looked back. The OP wants to install Virtualbox and already has some Virtualbox VMs.