hw/nvram/fw_cfg: fix FWCfgDataGeneratorClass::get_data() consumption

hw/nvram/fw_cfg: fix FWCfgDataGeneratorClass::get_data() consumption

The documentation on g_byte_array_free()
<https://developer.gnome.org/glib/stable/glib-Byte-Arrays.html#g-byte-array-free>
says:

> Returns
>
> the element data if free_segment is FALSE, otherwise NULL. The element
> data should be freed using g_free().

Because we currently call g_byte_array_free() with free_segment=TRUE, we
end up passing data=NULL to fw_cfg_add_file().

On the plus side, fw_cfg_data_read() and fw_cfg_dma_transfer() both deal
with NULL data gracefully: QEMU does not crash when the guest reads such
an item, the guest just gets a properly sized, but zero-filled blob.

However, the bug breaks UEFI HTTPS boot, as the IANA_TLS_CIPHER array,
generated otherwise correctly by the "tls-cipher-suites" object, is in
effect replaced with a zero blob.

Fix the issue by passing free_segment=FALSE to g_byte_array_free():

- the caller (fw_cfg_add_from_generator()) temporarily assumes ownership
  of the generated byte array,

- then ownership of the byte array is transfered to fw_cfg, as
  fw_cfg_add_file() links (not copies) "data" into fw_cfg.

Cc: "Daniel P. Berrangé" <berrange@redhat.com>
Cc: "Philippe Mathieu-Daudé" <philmd@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Fixes: 3203148917d035b09f71986ac2eaa19a352d6d9d
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200916151510.22767-1-lersek@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>