I/O test 151 testUnderLoad is flaky under AWS CI runners
## Host environment
- Operating system: Unknown
- OS/kernel version: Unknown
- Architecture: x86_64
- QEMU flavor: qemu-system-x86_64
- QEMU version: e89049b3ba5f1f0468bc0d294173345597514a1b
- QEMU command line:
```
check -qcow2 151
```
## Description of problem
Running the full set of I/O tests under the AWS CI runners results in a pretty reliable failure of 151
```
>>> RUST_BACKTRACE=1 PYTHON=/builds/qemu-project/qemu/build/pyvenv/bin/python3 MALLOC_PERTURB_=226 UBSAN_OPTIONS=halt_on_error=1:abort_on_error=1:print_summary=1:print_stacktrace=1 MESON_TEST_ITERATION=1 LD_LIBRARY_PATH=/builds/qemu-project/qemu/build/subprojects/libvfio-user/lib MSAN_OPTIONS=halt_on_error=1:abort_on_error=1:print_summary=1:print_stacktrace=1 ASAN_OPTIONS=halt_on_error=1:abort_on_error=1:print_summary=1 /builds/qemu-project/qemu/build/pyvenv/bin/python3 /builds/qemu-project/qemu/build/../tests/qemu-iotests/check -tap -qcow2 151 --source-dir /builds/qemu-project/qemu/tests/qemu-iotests --build-dir /builds/qemu-project/qemu/build/tests/qemu-iotests
――――――――――――――――――――――――――――――――――――― ✀ ―――――――――――――――――――――――――――――――――――――
stderr:
--- /builds/qemu-project/qemu/tests/qemu-iotests/151.out
+++ /builds/qemu-project/qemu/build/scratch/qcow2-file-151/151.out.bad
@@ -1,5 +1,14 @@
-.......
+......F
+======================================================================
+FAIL: testUnderLoad (__main__.TestLowThrottledWithNbdExport)
+Throttle the source node, then issue a whole bunch of external requests
+----------------------------------------------------------------------
+Traceback (most recent call last):
+ File "/builds/qemu-project/qemu/tests/qemu-iotests/151", line 393, in testUnderLoad
+ self.assertGreater(start_remaining - end_remaining, 0)
+AssertionError: 0 not greater than 0
+
----------------------------------------------------------------------
Ran 7 tests
-OK
+FAILED (failures=1)
(test program exited with status code 1
```
The same test works reliably on the GitLab shared runners.
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