virt-viewer: fix display of guest name in title for vnc
    
    The following commit broke the display of the guest name in
    the title for VNC displays:
    
      commit 61a1bc4dcbb056755fe96c5945f84c1312041059
      Author: Pavel Grunt <pgrunt@redhat.com>
      Date:   Wed Apr 15 13:50:35 2015 +0200
    
        session-vnc: Set window for display to avoid gtk-vnc v0.3.8 crash
    
        The VNC display widget of gtk-vnc v0.3.8 needs a window at the moment
    
    The problem is that this causes the window to be associated
    with the display before the guest name is available. Thus
    when ensure_window_for_display() runs, the window is already
    configured and so it never invokes the logic to set the title.
    
    The fix is to unconditionally update the title in the
    ensure_window_for_display() method, even if the window already
    exists.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
    (cherry picked from commit 35e489d5190f7acd822240e777c43a2432fbad7b)