Attempting to create an empty virtio-blk drive results into:
-device virtio-blk-pci,scsi=off,bus=pci.0,addr=0xc,drive=drive-virtio-disk1,id=virtio-disk1: Device needs media, but drive is empty

Attempting to eject media from virtio-blk based drive results into:
error: internal error: unable to execute QEMU command 'eject': Device 'drive-virtio-disk0' is not removable

Forbid configurations where users would attempt to use cdroms in virtio
bus.

Fix few wrong examples which are not really relevant to the tested code.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>