Just recently, qemu forbade specifying format for sourceless
disks (qemu commit 39c4ae941ed992a3bb5). It kind of makes sense.
If there's no file to open, why specify its format. Anyway, I
have a domain like this:

    <disk type='file' device='cdrom'>
      <driver name='qemu' type='raw'/>
      <target dev='hda' bus='ide'/>
      <readonly/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>

and obviously I am unable to start it. Therefore, a fix on our
side is needed too.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>