domain_conf: Resolve Coverity REVERSE_INULL

domain_conf: Resolve Coverity REVERSE_INULL

Coverity complains that checking for domain->def being non NULL in the
if (live) path of virDomainObjAssignDef() would be unnecessary or a
NULL deref since the call to virDomainObjIsActive() would already
dereference domain->def when checking if the def->id field was != -1.

Checked all callers to virDomainObjAssignDef() and each at some point
dereferences (vm)->def->{field} prior to calling when live is true.