trace/xen: add skeleton for Xen trace events

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

include/trace/events/xen.h

+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM xen
+
+#if !defined(_TRACE_XEN_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_XEN_H
+
+#include <linux/tracepoint.h>
+#include <asm/paravirt_types.h>
+
+#endif /*  _TRACE_XEN_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>