qemu: Specify format= iff disk source is not empty

qemu: Specify format= iff disk source is not empty

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: Michal Privoznik <mprivozn@redhat.com>