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>