vnc: ensure connection sharing/limits is always configured

The connection sharing / limits are only set in the
vnc_display_open() method and so missed when VNC is running
with '-vnc none'. This in turn prevents clients being added
to the VNC server with the QMP "add_client" command.

This was introduced in

  commit e5f34cdd2da54f28d90889a3afd15fad2d6105ff
  Author: Gerd Hoffmann <kraxel@redhat.com>
  Date:   Thu Oct 2 12:09:34 2014 +0200

      vnc: track & limit connections

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1470134726-15697-4-git-send-email-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
