x86/shutdown: add fall-through comment

Coverity ID: 1362037

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/shutdown.c
+++ b/xen/arch/x86/shutdown.c
@@ -541,6 +541,7 @@ void machine_restart(unsigned int delay_
         {
         case BOOT_INVALID:
             ASSERT_UNREACHABLE();
+            /* fall through */
         case BOOT_KBD:
             /* Pulse the keyboard reset line. */
             for ( i = 0; i < 100; i++ )