[Libvir] Question about specifications of virsh start

Hi I have a question about specifications of virsh start. It is necessary to remove "type=ioemu" from the configuration file of Xen, when HVM domain installed Para Virtualized Driver use unmodified driver on Xen 3.0.3. But,libvirt always adds "type=ioemu" to vif of HVM domain in xm_internal.c. Therefore virsh start cannot start correctly HVM domain installed Para Virtualized Driver. Could you teach why does libvirt disregard setting of configuration file and set type=ioemu ? Thanks, Masayuki Sunou.