xen/tools: tracing: trace (Credit2) runq traversal.

xen/tools: tracing: trace (Credit2) runq traversal.

When traversing a Credit2 runqueue to select the
best candidate vCPU to be run next, show in the
trace which vCPUs we consider.

A bit verbose, but quite useful, considering that
we may end up looking at, but then discarding, one
of more vCPU. This will help understand which ones
are skipped and why.

Also, add how much credits the chosen vCPU has
(in the TRC_CSCHED2_RUNQ_CANDIDATE record). And,
while there, fix a bug in tools/xentrace/formats
(still in the output of TRC_CSCHED2_RUNQ_CANDIDATE).

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