The timestamp typically will come from 'trace.py list' output.  This
change makes it easier to paste the timestamp. Listing prints the time
with '.' separating seconds from nanoseconds.  The options '--since'
and '--until' accept timestamp in nanoseconds so the dot must have
been removed. Now one only needs to append the 's' suffix which
changes the unit to seconds.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>