xen/manage: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/5cfc00b1d8ed68eb2c2b6317806a0aa7e57d27f1.1605896060.git.gustavoars@kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/manage.c

@@ -179,6 +179,7 @@ static int poweroff_nb(struct notifier_block *cb, unsigned long code, void *unus
 	case SYS_HALT:
 	case SYS_POWER_OFF:
 		shutting_down = SHUTDOWN_POWEROFF;
+		break;
 	default:
 		break;
 	}