Reduce the number of disks in virtio_scsi_cdrom installation

There are too many disks in virtio_scsi_cdrom installation for windows.
It results in seabios can not detect the install CD.
Signed-off-by: Nqingwangrh <qinwang@redhat.com>

qemu/tests/cfg/virtio_scsi_cdrom.cfg

@@ -72,6 +72,8 @@
                     image_size_equal = 1G
                     ahci:
                         images = "image1 stg stg2 stg3 stg4 stg5"
+                    default_bios..Windows:
+                        images = "image1 stg stg2 stg3 stg4 stg5"
             variants:
                 - aio_native:
                     image_aio = native