qemu.tests.cfg: Don't add any cdrom in multi_disk.max_disk

This test needs to add as many disk as qemu can accept, so
the cdrom added globally is unnecessary.
Signed-off-by: NQingtang Zhou <qzhou@redhat.com>

qemu/tests/cfg/multi_disk.cfg

@@ -20,6 +20,7 @@
             remove_image_stg = yes
             n_repeat = 10
         - max_disk:
+            cdroms = ""
             ide:
                 # 3 + boot disk = 4 disks
                 stg_image_num = 3