vnc: drop display+ws_display from VncDisplay

Nobody cares about those strings, they are only used to check whenever
the vnc server / websocket support is enabled or not.  Add bools for
this and drop the strings.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
