Slight redefinition of virtual operations holding assets

While there was no reason not to do it, only the trivial part of the task was done so it took very little time, and it also serves as a protection against passing wrong assets to virtual operations, the impact was extremely disappointing despite hundreds of millions of vops in database. Therefore even though we could still continue with more complex cases, we most likely won't because there would be no gain. It also puts into question somewhat similar but separate task of merging vops related to comment payouts - at the very least we'll have to estimate the impact before making any changes.