xen/pvcalls: introduce the pvcalls xenbus backend

Introduce a xenbus backend for the pvcalls protocol, as defined by
https://xenbits.xen.org/docs/unstable/misc/pvcalls.html.

This patch only adds the stubs, the code will be added by the following
patches.
Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
CC: boris.ostrovsky@oracle.com
CC: jgross@suse.com
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/pvcalls-back.c

+/*
+ * (c) 2017 Stefano Stabellini <stefano@aporeto.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#include <linux/kthread.h>
+#include <linux/list.h>
+#include <linux/radix-tree.h>
+#include <linux/module.h>
+#include <linux/semaphore.h>
+#include <linux/wait.h>
+
+#include <xen/events.h>
+#include <xen/grant_table.h>
+#include <xen/xen.h>
+#include <xen/xenbus.h>
+#include <xen/interface/io/pvcalls.h>
+
+static int pvcalls_back_probe(struct xenbus_device *dev,
+			      const struct xenbus_device_id *id)
+{
+	return 0;
+}
+
+static void pvcalls_back_changed(struct xenbus_device *dev,
+				 enum xenbus_state frontend_state)
+{
+}
+
+static int pvcalls_back_remove(struct xenbus_device *dev)
+{
+	return 0;
+}
+
+static int pvcalls_back_uevent(struct xenbus_device *xdev,
+			       struct kobj_uevent_env *env)
+{
+	return 0;
+}
+
+static const struct xenbus_device_id pvcalls_back_ids[] = {
+	{ "pvcalls" },
+	{ "" }
+};
+
+static struct xenbus_driver pvcalls_back_driver = {
+	.ids = pvcalls_back_ids,
+	.probe = pvcalls_back_probe,
+	.remove = pvcalls_back_remove,
+	.uevent = pvcalls_back_uevent,
+	.otherend_changed = pvcalls_back_changed,
+};