To be honest, the only practical reason for me to use KVM instead of VirtualBox (which I'm already using via Vagrant too) is I have more control over "low-level

To be honest, the only practical reason for me to use KVM instead of VirtualBox (which I'm already using via Vagrant too) is I have more control over "low-level" features, eg. I can setup VGA passthrough. But it seems they don't allow such features, and "don't have plans to allow the user to add any custom parameter to the VMs"? Great work nonetheless. Edit: I understand my use case is not exactly what they're looking for.