qemu-iotests/stream-under-throttle: do not shutdown QEMU

Without a kernel or boot disk a QEMU on s390 will exit (usually with a
disabled wait state). This breaks the stream-under-throttle test case.
Do not exit qemu if on s390.

Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Message-Id: <20221207131452.8455-1-borntraeger@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

tests/qemu-iotests/tests/stream-under-throttle

@@ -88,6 +88,8 @@ class TestStreamWithThrottle(iotests.QMPTestCase):
                            'x-iops-total=10000,x-bps-total=104857600')
         self.vm.add_blockdev(self.vm.qmp_to_opts(blockdev))
         self.vm.add_device('virtio-blk,iothread=iothr0,drive=throttled-node')
+        if iotests.qemu_default_machine == 's390-ccw-virtio':
+            self.vm.add_args('-no-shutdown')
         self.vm.launch()
 
     def tearDown(self) -> None: