Currently `-vnc none` is fatal when built with `--disable-vnc`.  Make
vnc_parse accept "none", so QEMU still run without using vnc.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Message-id: 20201009014032.3507-1-jandryuk@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>