Not sure about drag'n'drop. Also I've noticed that even when you're aware of the way USB passthrough in virt-manager GUI works that it seems to have some bugs.
I'm mostly interested in if I can use virtualbox accelerated video with kvm because virgl3d seems well behind in that area.
ah yes maybe drag'n'drop is not working I have no idea tbh but I don't remember it working reliably in virtualbox and shared folders always worked better in my limited experience.
Shared folders does indeed seem like a weak point for kvm/virt-manager. There's the virtioFS but this is a pretty recent addition that was also recently pretty buggy on Windows.
I'm not even sure what your alternatives were for this before now, I guess everyone was just using samba.
I'm mostly interested in if I can use virtualbox accelerated video with kvm because virgl3d seems well behind in that area.