tests/qtest/boot-serial-test: Correct HPPA machine name

tests/qtest/boot-serial-test: Correct HPPA machine name

Commit 7df6f751176 ("hw/hppa: Split out machine creation")
renamed the 'hppa' machine as 'B160L', but forgot to update
the boot serial test, which ended being skipped.

Cc: qemu-stable@nongnu.org
Fixes: 7df6f751176 ("hw/hppa: Split out machine creation")
Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20250102100340.43014-2-philmd@linaro.org>