[libvirt] how to assign an ip when assign a NIC or VF to the Virtual Machine?

Hi， I use the *Intel Virtualization Technology for Directed I/O*(VT-d) or *Single-Root I/O Virtualization*(SR-IOV) to assign a network card or Virtual Function(VF) to a Virtual Machine(VM) to access the network by the hostdev element in libvirt Domain XML. But the ethX in the VM hasn't an ip,I must to enter the VM to assign an ip to the VM manually. So how can I assign an ip to the VM when assign the NIC or VF to the VM ?thanks. best regards, qinguan