qemu.tests.multi_disk: Fix bootindex bug

Fix signal_repeat variant to boot with the correct bootindex.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/cfg/multi_disk.cfg

@@ -13,6 +13,7 @@
             image_format_stg = qcow2
             image_name_stg = storage
             image_size_stg = 1G
+            image_boot_stg = no
             n_repeat = 10
         - max_disk:
             only virtio_blk