tests/libqos: Add pc_fw_cfg_uninit() and use it

tests/libqos: Add pc_fw_cfg_uninit() and use it

The pc_fw_cfg_init() function allocates an IO QFWCFG object.
Add the pc_fw_cfg_uninit() function to deallocate it (and use it).

Signed-off-by: Li Qiang <liq3ea@163.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20190424140643.62457-2-liq3ea@163.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[PMD: Split patch, fill commit description, call uninit in malloc-pc.c]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>