[libvirt] KVM/QEMU -net_user host and dns addr

Hi, I am using libvirt with kvm/qemu. And Im using -net_user network option of kvm/qemu. In order to send the dns addr and host addr I have to modify the xml by my slef. I am wondering why there is no built in function in libvirt api to access/modify dns addr and host addr from within libvirt under user mode network of kvm/qemu (-net_user). Is there a reason for not having this feature or is it just not been implemented? Chet