A real solution is how Qubes implements this. Unfortunately it's very
hard to implement, so that's out of scope for now.
learn why Qubes does "secure copy and paste"
learn how Qubes implements it from a usability perspective (global copy/paste: ctrl + shift + c | ctrl + shift + v)
rehash and post feature requests against VirtualBox and KVM
enable clipboard sharing by default in VirtualBox
@HulaHoop's decision, enabling/disabling clipboard sharing by default in KVM