bootdevice: add check in restore_boot_order()

qemu_boot_set() can't fail in restore_boot_order(),
then simply assert it doesn't fail, by passing
&error_abort if boot_set_handler set.
Suggested-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NGonglei <arei.gonglei@huawei.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>

bootdevice.c

@@ -105,7 +105,9 @@ void restore_boot_order(void *opaque)
         return;
     }
 
-    qemu_boot_set(normal_boot_order, NULL);
+    if (boot_set_handler) {
+        qemu_boot_set(normal_boot_order, &error_abort);
+    }
 
     qemu_unregister_reset(restore_boot_order, normal_boot_order);
     g_free(normal_boot_order);