virttest: Update cdrom image file name

The image_name for cdroms in our cfg files should be cdrom. So
need to update it, otherwise the cdrom device without file
will use the image_name defined for hard disk.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1141 from ypu/cdrom

virttest: Update cdrom image file name