sim: Add a SimObject python field which overrides the default c++ base.

sim: Add a SimObject python field which overrides the default c++ base.

The base for the c++ version of python SimObject classes is normally
inferred from the c++ version of the python base. There are some
specific cases where that isn't desired. This change makes it possible
to override the default behavior.

Change-Id: I2438dad767e2f56823bad42b3e6c7714ce97ef79
Reviewed-on: https://gem5-review.googlesource.com/10662
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>