sim: Prepare C++ side for Python 3

sim: Prepare C++ side for Python 3

Python 3 uses wide strings instead of ordinary strings for many
APIs. Add the necessary conversions to comply with the new API.

Change-Id: I6f45c9c532537d50d54b542f34eb8fd8cb375874
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15977
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>