Disabe multi_disk.all_drive_format_types on s390

The s390x supported drive formats is rather limited so it does not make
sense to even pretend to support this test. Let's just filter it out.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/cfg/multi_disk.cfg

@@ -48,6 +48,7 @@
         - all_drive_format_types:
             # virtio-scsi driver support from RHEL6.3
             no RHEL.3 RHEL.4 RHEL.5 RHEL.6.0 RHEL.6.1 RHEL.6.2
+            no s390x
             stg_image_num = 4
             stg_image_size = 100M
             stg_params = "drive_format:ide,scsi,virtio,scsi-hd,usb2"