hw/nvram/fw_cfg: Store 'reboot-timeout' as little endian

hw/nvram/fw_cfg: Store 'reboot-timeout' as little endian

The current codebase is not specific about the endianess of the
fw_cfg 'file' entry 'reboot-timeout'.

Per docs/specs/fw_cfg.txt:

  === All Other Data Items ===

  Please consult the QEMU source for the most up-to-date
  and authoritative list of selector keys and their respective
  items' purpose, format and writeability.

Checking the git history, this code was introduced in commit
ac05f3492421, very similar to commit 3d3b8303c6f8 for the
'boot-menu-wait' entry, which explicitely use little-endian.

OVMF consumes 'boot-menu-wait' as little-endian, however it does
not consume 'reboot-timeout'.

Regarding the git history and OVMF use, we choose to explicit
'reboot-timeout' endianess as little-endian.

Signed-off-by: Li Qiang <liq3ea@163.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190424140643.62457-4-liq3ea@163.com>
[PMD: Reword commit description based on review comments]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>