A running VM got its new CD-ROM on SCSI, the one bus the panel could plug
in then, and Windows does not see a virtio-scsi drive before the driver
on that ISO is installed. Add CD-ROM is now a dialog that chooses the bus
and the ISO: a running VM takes USB, first, which every guest sees and
which hot-plugs, or SCSI; a shut-off one SATA (IDE on pc), SCSI or USB.
libvirt has no empty USB drive, so a USB CD-ROM takes its ISO when it is
added and is detached instead of ejected, also while the VM runs.
A USB CD-ROM is a CD drive on QEMU's usb-bot only. Where QEMU has none
(RHEL 9), libvirt took usb-storage, which shows the guest a hard disk
that Windows does not read; the drive now asks for usb-bot, and such a
host refuses it with libvirt's message.
