qom: Fix default values in help

Output of default values in device help is broken:

    $ ./qemu-system-x86_64 -S -display none -monitor stdio
    QEMU 5.2.50 monitor - type 'help' for more information
    (qemu) device_add pvpanic,help
    pvpanic options:
      events=<uint8>         -  (default: (null))
      ioport=<uint16>        -  (default: (null))
      pvpanic[0]=<child<qemu:memory-region>>

The "(null)" is glibc printing a null pointer.  Other systems crash
instead.  Having a help request crash a running VM can really spoil
your day.

Root cause is a botched replacement of qstring_free() by
g_string_free(): to get the string back, we need to pass true to the
former, but false to the latter.  Fix the argument.

Fixes: eab3a4678b
Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210324084130.3986072-1-armbru@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

qom/object_interfaces.c

@@ -159,7 +159,7 @@ char *object_property_help(const char *name, const char *type,
     }
     if (defval) {
         g_autofree char *def_json = g_string_free(qobject_to_json(defval),
-                                                  true);
+                                                  false);
         g_string_append_printf(str, " (default: %s)", def_json);
     }