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/Include/Protocol/XenBus.h

@@ -198,6 +198,24 @@ XENSTORE_STATUS
   IN BOOLEAN                Abort
   );
 
+/**
+  Set a new state for the frontend of the PV driver.
+
+  @param This         A pointer to XENBUS_PROTOCOL instance.
+  @param Transaction  The transaction to end/commit.
+  @param State        The new state to apply.
+
+  @return  On success, XENSTORE_STATUS_SUCCESS. Otherwise an errno value
+           indicating the type of failure.
+**/
+typedef
+XENSTORE_STATUS
+(EFIAPI *XENBUS_SET_STATE)(
+  IN XENBUS_PROTOCOL        *This,
+  IN XENSTORE_TRANSACTION   Transaction,
+  IN XenBusState            State
+  );
+
 /**
   Grant access to the page Frame to the domain DomainId.
 
@@ -322,6 +340,7 @@ struct _XENBUS_PROTOCOL {
   XENBUS_XS_REMOVE              XsRemove;
   XENBUS_XS_TRANSACTION_START   XsTransactionStart;
   XENBUS_XS_TRANSACTION_END     XsTransactionEnd;
+  XENBUS_SET_STATE              SetState;
 
   XENBUS_GRANT_ACCESS           GrantAccess;
   XENBUS_GRANT_END_ACCESS       GrantEndAccess;