domain_conf: Format <defaultiothread/> more often

domain_conf: Format <defaultiothread/> more often

The <defaultiothread/> element is formatted inside
virDomainDefaultIOThreadDefFormat() which is called only from
virDomainDefIOThreadsFormat() (so that IOThread related stuff is
formatted calling one function). However, when there are no
<iothreadids/> defined (or only autoallocated ones are present),
then the outer formatting function exits early never calling the
<defaultiothread/> formatter.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>