ui: refactor code for populating SocketAddress from vnc_display_open

The code which interprets the CLI args to populate the SocketAddress
objects for plain & websockets VNC is quite complex already and will
need further enhancements shortly. Refactor it into separate methods
to avoid vnc_display_open getting even larger. As a side effect of
the refactoring, it is now possible to specify a listen address for
the websocket server explicitly. e.g,

  -vnc localhost:5900,websockets=0.0.0.0:8080

will listen on localhost for the plain VNC server, but expose the
websockets VNC server on the public interface. This refactoring
also removes the restriction that prevents enabling websockets
when the plain VNC server is listening on a UNIX socket.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170203120649.15637-5-berrange@redhat.com

[ kraxel: squashed clang build fix ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>