Provide a stub acpi_md_sleep_init on xen, pointed out by Paul Goyette

on current-users.

sys/arch/xen/xen/xen_acpi_machdep.c

@@ -1,4 +1,4 @@
-/*	$NetBSD: xen_acpi_machdep.c,v 1.1 2006/04/09 19:28:01 bouyer Exp $	*/
+/*	$NetBSD: xen_acpi_machdep.c,v 1.2 2007/12/15 19:24:17 jmcneill Exp $	*/
 
 #include "acpi.h"
 
@@ -13,3 +13,10 @@ acpi_md_sleep(int state)
 	printf("acpi: sleep not implemented\n");
 	return (-1);
 }
+
+void
+acpi_md_sleep_init(void)
+{
+	/* Not supported on xen */
+	return;
+}