Fix build when building without oVirt
    
    As remote_viewer_iso_list_dialog_new() is defined on
    remote-viewer-iso-list-dialog.h which is only build with oVirt
    integration.
    
     > undefined reference to `remote_viewer_iso_list_dialog_new'
    
    Note that the callback virt_viewer_window_menu_change_cd_activate() is
    only visible if oVirt is built in.
    
    Signed-off-by: Victor Toso <victortoso@redhat.com>