[libvirt-users] Hello ,How Can I do USB emulation

I have a usb.img file, and i want to create a usbdisk(USB emulation) for vpc in a kvm server. I found something like this: qemu-system-x86_64 -device piix3-usb-uhci \ -drive id=my_usb_disk,file=usbdisk.img,if=none \ -device usb-storage,drive=my_usb_disk but i want to do it in a xml files for libvirt. i did not find any example for it. how can i do it thank u very much.