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.