This is only half of the work, because the proxy devices (virtio-*-pci,
virtio-*-ccw, etc.) are still included unconditionally. It is still a
move in the right direction.
Based-on: <20180522194943.24871-1-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
