OvmfPkg/XenBusDxe: Introduce XenBus support itself.

This is a bus-like on top of XenStore. It will look for advertised
ParaVirtualized devices and initialize them by producing XenBus
protocol.

Change in V4:
- Replace the license by the commonly used file header text.
- Clean XenBus.h header (remove copyright that does not belong to the
  file anymore; and rewrite the brief description of the file)
- Fix description on the function

Change in V3:
- Insert to ChildList later, once populated.
- Remove XENBUS_XENSTORE_NODE macro.
- add comment to XenBusAddDevice and XenBusEnumerateBus about
  concurrency calls.
- Add a description to the introduced member to the protocol.

Change in V2:
- comment, file header
- Fix comment style
- Error handling in the main init function
- coding style
- Fix error path in add device.

Origin: FreeBSD 10.0
License: This patch adds XenBus.c which is under the MIT licence.
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16270 6f19259b-4bc3-4df7-8a09-765794883524

OvmfPkg/XenBusDxe/XenBus.h

@@ -0,0 +1,44 @@
+/** @file
+  XenBus Bus driver declarations.
+
+  Copyright (C) 2014, Citrix Ltd.
+
+  This program and the accompanying materials
+  are licensed and made available under the terms and conditions of the BSD License
+  which accompanies this distribution.  The full text of the license may be found at
+  http://opensource.org/licenses/bsd-license.php
+
+  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
+  WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+
+**/
+#ifndef _XEN_XENBUS_XENBUSB_H
+#define _XEN_XENBUS_XENBUSB_H
+
+#include "XenBusDxe.h"
+
+#define XENBUS_DEVICE_PATH_TYPE_VBD 0x1
+struct _XENBUS_DEVICE_PATH {
+  VENDOR_DEVICE_PATH  Vendor;
+  UINT8               Type;
+  UINT16              DeviceId;
+};
+
+
+/**
+  Perform XenBus bus enumeration and install protocol for children.
+
+  Caller should ensure that it is the only one to call this function. This
+  function cannot be called concurrently.
+
+  @param Dev   A XENBUS_DEVICE instance.
+
+  @return      On success, XENSTORE_STATUS_SUCCESS. Otherwise an errno value
+               indicating the type of failure.
+**/
+XENSTORE_STATUS
+XenBusEnumerateBus (
+  XENBUS_DEVICE *Dev
+  );
+
+#endif /* _XEN_XENBUS_XENBUSB_H */