Observations
(1) A new virtual disk can be hot plugged into a KVM/qemu VM as a qemu virtio device, e.g. see question KVM and Libvirt - How do I hotplug a new virtio disk?. However, this requires PCI hot-plugging support from the guest OS and this is not always available.
(2) Hot plugging of USB storage is well supported in many systems and there are guides how to forward a physical USB disk to a guest VM, e.g. https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/virtualization_administration_guide/sect-managing_guest_virtual_machines_with_virsh-attaching_and_updating_a_device_with_virsh.
Question
Can kvm/qemu's virtual USB system be used to attach a virtual disk (image file or a block device that is not a USB device) as a USB storage device to the guest VM so that it appears in the active VM?
