virsh: Preserve startupPolicy in change-media command
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1250331
    
    Even after my rework of startupPolicy handling, one command
    slipped my attention. The change-media command has a very unique
    approach to constructing disk XML. However, it will not preserve
    startupPolicy attribute.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>