trace-cmd library: Set correct CPU to the record, retrieved with tracecmd_peek_data

This clean up is needed for the design of the next version of the trace
file, where only CPUs with trace data are stored in the file - empty CPUs
are omitted. Changed tracecmd_peek_data() to use the CPU id, instead of
the CPU index.

Link: https://lore.kernel.org/linux-trace-devel/20210914131232.3964615-19-tz.stoyanov@gmail.com

Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>