test harness hang running functional test

[Peter Maydell]

I was trying to track down why one of the functional tests was
hanging, so I tried running it directly:

UBSAN_OPTIONS=halt_on_error=1:abort_on_error=1
QEMU_TEST_QEMU_BINARY=build/clang/qemu-system-loongarch64
PYTHONPATH=./build/clang/pyvenv:./python time
./build/clang/pyvenv/bin/python3
./tests/functional/test_loongarch64_virt.py

(This is with a QEMU built with the undefined-behaviour
sanitizer.)

Mostly this seems to succeed, but this time it's hung. Looking
at the process tree:

petmay01 3616444  0.0  0.0   4768  1124 pts/2    S+   15:53   0:00
 \_ time ./build/clang/pyvenv/bin/python3
./tests/functional/test_loongarch64_virt.py
petmay01 3616445 99.8  0.0  38480 23620 pts/2    R+   15:53  13:20
     \_ ./build/clang/pyvenv/bin/python3
./tests/functional/test_loongarch64_virt.py
petmay01 3616448  0.0  0.0      0     0 pts/2    Z+   15:53   0:00
         \_ [qemu-system-loo] <defunct>

The QEMU process itself has exited, but the test harness has
not yet noticed or reaped the zombie process. Instead it's sitting
there eating CPU. Presumably this is a bug in the test harness?

thanks
-- PMM