If the ``-gdb`` option is not used, ``$GDB_OPTIONS`` is ignored,
regardless of whether it is set or not.
+* ``-valgrind`` attaches a valgrind instance to QEMU. If it detects
+ warnings, it will print and save the log in
+ ``$TEST_DIR/<valgrind_pid>.valgrind``.
+ The final command line will be ``valgrind --log-file=$TEST_DIR/
+ <valgrind_pid>.valgrind --error-exitcode=99 $QEMU ...``
+
* ``-d`` (debug) just increases the logging verbosity, showing
for example the QMP commands and answers.