fw_cfg: Make qemu_extra_params_fw locally

fw_cfg: Make qemu_extra_params_fw locally

qemu_extra_params_fw[] has external linkage, but is used
only in fw_cfg_bootsplash(), it makes sense to make it
locally.

Signed-off-by: Li Qiang <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Reviewed-by: Gerd Hoffmann <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <1542777026[email protected]>
[PMD: Removed qemu_extra_params_fw declaration in vl.c]
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>