virt.shared.cfg: force cd_format in machines.cfg

force cd_format for unattended_install with virtio_scsi or virtio_blk cdrom
test, because guest need special driver to support it before bootup.

Signed-off-by: Xu Tian <xutian@redhat.com>
Merge pull request #1543 from xutian/fix_machines

virt.shared.cfg: force cd_format in machines.cfg