Remove obsolete use of SpiceChannel:virt-viewer-display object data
    
    Commit 0d58d9c72 removed the setting of the
    SpiceChannel:virt-viewer-display object data, but there was still a
    call to g_object_get_data() trying to use it. Since it's only used to
    output a debug log, we can remove this call and fix up the debug log.