I think there is no good reason to index virtual machine images. I added it to my configuration manually already.
Why not? I may have stored some VM image in one of my projects folders and no longer remember which one. So it should be indexed. Indexing a VM image only takes up a few dozens of bytes in the DB. There are plenty of complaints from users where baloo skips a file. We should not any more reasons to complain.
I experienced baloo repeatedly trying to index a qcow file *while* I was using the VM, causing considerably extra load on the system. This is with Baloo 5.70. Has this been fixed meanwhile? If not, I'd file a new bug about that.