conf: Skip post parse callbacks when creating copy
    
    When creating a copy of virDomainDef we save ourselves the
    trouble of writing deep-copy functions and just format and parse
    back domain/device XML. However, the XML we are parsing was
    already fully formatted - there is no reason to run post parse
    callbacks (which fill in blanks - there are none!).
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>