trace: Fix overutilized duration
trace: Fix overutilized duration

The previous implementation set the `len` field of the last overutilized
event to NaN. This commit sets that duration to:
trace_end - event_timestamp

This is mostly to fix erroneous overutilized plotting: if there was
a single overutilized=1 event early in a trace and no other overutilized
event later on, we wouldn't plot the system as overutilized.

ARM-software/lisa,credp/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,bjackman/lisa,credp/lisa,credp/lisa,ARM-software/lisa,bjackman/lisa