system/qtest: Clean up global variable shadowing in qtest_server_init()

system/qtest: Clean up global variable shadowing in qtest_server_init()

Rename the variable to fix:

  softmmu/qtest.c:869:13: error: declaration shadows a variable in the global scope [-Werror,-Wshadow]
      Object *qtest;
              ^
  softmmu/qtest.c:53:15: note: previous declaration is here
  static QTest *qtest;
                ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20231009100251.56019-2-philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>