fw_cfg: fix the life cycle and the name of "qemu_extra_params_fw"

fw_cfg: fix the life cycle and the name of "qemu_extra_params_fw"

Commit 19bcc4bc3213 ("fw_cfg: Make qemu_extra_params_fw locally",
2019-01-04) changed the storage duration of the "qemu_extra_params_fw"
array from static to automatic. This broke the interface contract on the
fw_cfg_add_file() function, which is documented as follows, in
"include/hw/nvram/fw_cfg.h":

> [...] The data referenced by the starting pointer is only linked, NOT
> copied, into the data structure of the fw_cfg device. [...]

As a result, when guest firmware fetches the "etc/boot-menu-wait" fw_cfg
file, it now sees garbage. Fix the regression by changing the storage
duration to allocated. (The call is reached at most once, on the realize
path of the board-specific fw_cfg sysbus device.)

While at it, clean up the name and the assignment of the object as well.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Fixes: 19bcc4bc3213e78c303ad480a7a578f62258252d
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>