xen: make tracebuffer configurable

xen: make tracebuffer configurable

Xen internal running status(trace event at pre-defined trace point)
will be saved to trace memory when enabled.
Trace event data and config params can be read/changed
by system control hypercall at run time.

Can be disabled for smaller code footprint.

Signed-off-by: Baodong Chen <chenbaodong@mxnavi.com>
Acked-by: George Dunlap <george.dunlap@citrix.com> [tracing]
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>