https://www.reddit.com/r/kde/comments/6uv167/is_there_a_kde_equivalent_to_gnome_boxes/

Tldr - Virtualbox uses qt5 and virt manager is also supported

The “no official support” thing is really only there because I don’t drive X11 and don’t test for bugs there myself. Anecdotally, it works about the same feature/bug-wise.