xen/tools: tracing: improve tracing of context switches.

xen/tools: tracing: improve tracing of context switches.

Right now, two out of the three events related to
context switch (that is TRC_SCHED_SWITCH_INFPREV and
TRC_SCHED_SWITCH_INFNEXT) only report the domain id,
and not the vcpu id.

That's omitting a useful piece of information, and
even if we be figured that out by looking at other
records, that's unnecessarily complicated (especially
if working on a trace from a sctipt).

This changes both the tracing code in Xen and the parsing
code in tools at once, to avoid introducing transitional
regressions.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>