Revert "conf: Skip post parse callbacks when creating copy"

Revert "conf: Skip post parse callbacks when creating copy"

This breaks vCPU hotplug, because when starting a domain, we
create a copy of domain definition (which becomes live XML) and
during the post parse callbacks we might adjust some tunings so
that vCPU hotplug is possible.

This reverts commit 581b7756af18dcf84b57d9947978725d2dfbfc18.