From virsh:
virsh # qemu-monitor-command --hmp DOMAIN drive_add 0 if=none,file=/tmp/mydisk.img,id=usbdrive1 virsh # qemu-monitor-command --hmp DOMAIN info block [...] usbdrive1 (#block9999): /tmp/mydisk.img (raw) Attached to: /machine/peripheral/usbdevice/usbdevice.0/legacy[0] Cache mode: writethrough virsh # qemu-monitor-command --hmp DOMAIN device_add usb-storage,id=usbstorage1,drive=usbdrive1