iotests: make 235 work on s390 (and others)

"-machine pc" will not work all architectures. Lets fall back to the
default machine by not specifying it.

In addition we also need to specify -no-shutdown on s390 as qemu will
exit otherwise.

Cc: qemu-stable@nongnu.org
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 2c26e648e4)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

tests/qemu-iotests/235

@@ -49,7 +49,9 @@ qemu_img_create('-f', iotests.imgfmt, '-o', 'preallocation=metadata', disk,
                 str(size))
 
 vm = QEMUMachine(iotests.qemu_prog)
-vm.add_args('-machine', 'pc,accel=kvm')
+vm.add_args('-machine', 'accel=kvm')
+if iotests.qemu_default_machine == 's390-ccw-virtio':
+        vm.add_args('-no-shutdown')
 vm.add_args('-drive', 'id=src,file=' + disk)
 vm.launch()