xen: credit2: issues in csched2_cpu_pick(), when tracing is enabled.

xen: credit2: issues in csched2_cpu_pick(), when tracing is enabled.

In fact, when not finding a suitable runqueue where to
place a vCPU, and hence using a fallback, we either:
 - don't issue any trace record (while we should, at
   least, output the chosen pcpu),
 - risk underruning when accessing the runqueues
   array, while preparing the trace record.

Fix both issues and, while there, also a couple of style
problems found nearby.

Spotted by Coverity.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>