python: Fix param -> int conversion issues

python: Fix param -> int conversion issues

Python 3 doesn't convert params to integers automatically in
range(). Add __index__ to CheckedInt to enable implicit conversions
again. Add explicit conversions where necessary.

Change-Id: I2de6c9906d3bb7616f12ada6728b9e4b1928511c
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/16000
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>