There have been many similar detections in virtual image files, and it is possible that because of its sheer size and the way they work it is entirely possible that there might be a string detected as a virus signature.
Preasuably you still run avast withing the VM Software ?
If so then this is less of an issue (as outlined above) or avast would also have detected it there.
Commonly we suggest that you exclude the *.vdmk file type in the avast Program Settings, Exclusions, so they aren't scanned by the on-demand scan (Simple User Interface).
