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
ac05f349, very similar to commit 3d3b8303 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: NLi Qiang <liq3ea@163.com>
Tested-by: NThomas Huth <thuth@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190424140643.62457-4-liq3ea@163.com>
[PMD: Reword commit description based on review comments]
Signed-off-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
(cherry picked from commit 04da9735)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

hw/nvram/fw_cfg.c

@@ -178,6 +178,7 @@ static void fw_cfg_reboot(FWCfgState *s)
 {
     const char *reboot_timeout = NULL;
     int64_t rt_val = -1;
+    uint32_t rt_le32;
 
     /* get user configuration */
     QemuOptsList *plist = qemu_find_opts("boot-opts");
@@ -194,7 +195,8 @@ static void fw_cfg_reboot(FWCfgState *s)
         }
     }
 
-    fw_cfg_add_file(s, "etc/boot-fail-wait", g_memdup(&rt_val, 4), 4);
+    rt_le32 = cpu_to_le32(rt_val);
+    fw_cfg_add_file(s, "etc/boot-fail-wait", g_memdup(&rt_le32, 4), 4);
 }
 
 static void fw_cfg_write(FWCfgState *s, uint8_t value)