it claimed to run out of space for virus definition updates (despite the fact that there was plenty of room on the drive)
As has been explained several times already, this has nothing to do with diskspace but with how that avast version was build. It simply can't handle the large database files.
because you certainly cannot count on its own uninstaller to truly get rid of it.
True. aswclear doesn't remove everything only some major parts/registry entries.
Revo uninstaller has been working on it for several hours
Strange. Revo shouldn't take more than a few minutes. Even on slow systems.

avast versions 8.0.1497 and lower are not supported anymore.
https://blog.avast.com/2015/05/06/support-for-older-avast-windows-consumer-version-will-end/

What Windows version are you running in the VM ?

On a side note:
It is unfortunately true that uninstalling through control panel in almost all cases leave things behind.
The same is true for the uninstallers provided by the developers.