vnc: remove vnc_display global

Replace with a vnc_displays list, so we can have multiple vnc server
instances.  Add vnc_server_find function to lookup a display by id.
With no id supplied return the first vnc server, for backward
compatibility reasons.

It is not possible (yet) to actually create multiple vnc server
instances.

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