qom: Fix default values in help

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: eab3a4678b07267c39e7290a6e9e7690b1d2a521
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>