cpu: Fix TrafficGen trace playback

cpu: Fix TrafficGen trace playback

This patch addresses an issue with trace playback in the TrafficGen
where the trace was reset but the header was not read from the trace
when a captured trace was played back for a second time. This resulted
in parsing errors as the expected message was not found in the trace
file.

The header check is moved to an init funtion which is called by the
constructor and when the trace is reset. This ensures that the trace
header is read each time when the trace is replayed.

This patch also addresses a small formatting issue in a panic.