tests/cdrom-test: Clean up global variable shadowing in prepare_image()

Rename the variable to fix:

  tests/qtest/cdrom-test.c:40:50: error: declaration shadows a variable in the global scope [-Werror,-Wshadow]
  static int prepare_image(const char *arch, char *isoimage)
                                                   ^
  tests/qtest/cdrom-test.c:18:13: note: previous declaration is here
  static char isoimage[] = "cdrom-boot-iso-XXXXXX";
              ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-ID: <20231009100251.56019-5-philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>