Hi GermanTux!
Kwin is literally developed by three people.
That's not an excuse for not reporting a bug to their bugtracker (assuming they do have one - I still haven't looked into it).
VirtualBox has other issue than dealing with minor KDE window manager bugs.
If it's a "minor KDE window manager bug", than that's not a problem for which Virtualbox' developers need to worry.
That's the price you pay for a really good, free, cross platform virtualizer.
...and a really good, free, cross platform (somehow) DE!

Bye
Cris