libs/utils/trace_analysis: fix for image file name
Created by: Leo-Yan
trace_analysis automatically saves image for method trace_analysis::plotTasks(), which conjunct task name for saving image file. But sometimes task name has special characters (like ':', '/'), so Linux cannot support these characters for file name.
So this patch is to fix this failure to convert special characters to underline '_'.
Signed-off-by: Leo Yan leo.yan@linaro.org