xen/tools: tracing: credits can go negative, so use int.

xen/tools: tracing: credits can go negative, so use int.

For Credit2, in both the trace records, inside Xen,
and in their parsing, in xenalyze.

In fact, as it is quite a bit better, in order to
understand how much negative credits have gone for
a certain vCPU, to see an actual negative number,
as compared to a wrapped around unsigned!

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