x86/64: add xen DomU support

Xen support for x86/generic was added in 1d6879ee.  This commit also
enables it for x86/64.

This was successfully tested with Xen 4.5, although the serial console
is broken in the same way as x86/generic (see FS#787)

Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>

target/linux/x86/64/base-files/lib/preinit/45_mount_xenfs

@@ -0,0 +1,9 @@
+#!/bin/sh
+# Copyright (C) 2010 OpenWrt.org
+
+do_mount_xenfs() {
+	grep -q xenfs /proc/filesystems && \
+		mount -o noatime -t xenfs none /proc/xen
+}
+
+boot_hook_add preinit_mount_root do_mount_xenfs