virt-manager is a great vm manager with kde compatibility, i use it all the time
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
There is a Qt interface for libvirt, called qt-virt-manager. This is a rough analog of virt-manager. Unfortunately, it hasn't seen a release since August 2021, nor a git commit since the same time. The good news is that it's written in C++: if you're so inclined, you ought to be able to get up to speed at least a bit faster than you would with a project in another language.
As much as I'd like a KDE-native VM GUI, I still just use virt-manager since it's maintained, and gets the job done. Generally I'm just using ssh to access VMs anyway, so it hardly matters. But if qt-virt-manager made a comeback, I'd definitely give it strong consideration.
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