run.py: allow multiple arguments to --trace

Allow to pass --trace multiple times to run.py so that the command
line interface is the same as for OSv's loader, eg:

  scripts/run.py --trace sched_wait* --trace memory_*

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>