ui: extract code to connect/listen from vnc_display_open

The code which takes a SocketAddress and connects/listens on the
network is going to get more complicated to deal with multiple
listeners. Pull it out into a separate method to avoid making the
vnc_display_open method even more complex.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170203120649.15637-6-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>