-drive if=none,format=qcow2,id=usb1,file=/var/lib/vz/images/100/vm-100-disk-1.qcow2 -device usb-storage,drive=usb1
args: -device piix3-usb-uhci,addr=0x18 -drive id=my_usb_disk,file=/my-zfs-pool/images/110/vm-110-disk-1.raw,if=none -device usb-storage,id=my_usb_disk,drive=my_usb_disk