The virtual file is somewhat large and complex and I guess it is possible that a string could match a signature. Many suggest that you exclude the virtual drive file from on-demand scans, which presumably was when this was detected ?
It is also possible that there might have been a virus in the virtual environment, the problem is that the actual image is one file when scanned by the on-demand scanner so it can't differentiate from a file within a virtual drive image.
You could add the following "*.vhc" and "*.vmc" (without the " quotes " ) to the avast Program Settings, Exclusions, Add, see image. That would exclude those file types the wildcard * means you don't have to name the file or folder or drive they are in.