Is there a way to keep an old Android installation as virtual machine?
ADB backs up Settings + Apps(System + User) + Apps Data + User Data. That's it. The backup made through it doesn't come close to be related to booting it into virtual machine or even a real device. Short story, you have a backup definitely but that's not a system image.

If you want to create a system image, you need to do it through a Nandroid backup or a custom tool. For the former, you require a custom recovery installed. But even that backup cannot be virtualized on a PC, not that I'm aware of any of yet.

However, you can virtualize (running real actually) this Nandroid backup using MultiRom. I did not use MultiRom but I found that folks use it to boot multiple Android OS. I'm yet to be certain whether this backup can be installed in a seconday ROM, I suppose it should be.
Besides, the last time I checked for Android virtualization, I got Android 4.3 as the latest available iso when 5.0 for devices was already launched. 

Note: This is an attempt and is far away from a good answer. Nonetheless, it doesn't qualify to be a comment either.