random copy-before-write iotest failure

[Peter Maydell]

This is on ppc64 (big-endian), a random failure
(it was while testing the riscv pullreq, but that doesn't touch
any of the block stuff):

616/635 qemu:block / qemu-iotests qcow2
           ERROR
      101.88s   exit status 1
――――――――――――――――――――――――――――――――――――― ✀  ―――――――――――――――――――――――――――――――――――――
stderr:
--- /home/pm215/qemu/tests/qemu-iotests/tests/copy-before-write.out
+++ 
/home/pm215/qemu/build/all/tests/qemu-iotests/scratch/copy-before-write/copy-before-wri
te.out.bad
@@ -1,5 +1,21 @@
-....
+..F.
+======================================================================
+FAIL: test_timeout_break_guest (__main__.TestCbwError.test_timeout_break_guest)
+----------------------------------------------------------------------
+Traceback (most recent call last):
+  File "/home/pm215/qemu/tests/qemu-iotests/tests/copy-before-write",
line 200, in test_ti
meout_break_guest
+    self.assertEqual(log, """\
+AssertionError: 'wrot[90 chars])\nwrote 524288/524288 bytes at offset
524288\[151 chars]c)\n' != 'wrot[90 chars])\nwrite failed: Connection
timed out\nread 10[85 chars]c)\n'
+  wrote 524288/524288 bytes at offset 0
+  512 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
++ write failed: Connection timed out
+- wrote 524288/524288 bytes at offset 524288
+- 512 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+  read 1048576/1048576 bytes at offset 0
+  1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+
+
 ----------------------------------------------------------------------
 Ran 4 tests

-OK
+FAILED (failures=1)

thanks
-- PMM