conf: Skip post parse callbacks when creating copy

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>