python: Prevent Python wrappers from deleting SimObjects

python: Prevent Python wrappers from deleting SimObjects

The PyBind wrappers could potentially delete SimObjects if they don't
have any references. This is not desirable since there could be
pointers to such objects within the C++ world. This problem doesn't
normally occur since Python typically holds a pointer to the root node
as long as the simulator is running.

Prevent SimObject and Param deletion by using a PyBind-prescribed
unique_ptr with a dummy deleter as the pointer wrapper for the Python
world.

Change-Id: Ied14602c9ee69a083a69c5dae1b5fcf8efb4548a
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3224
Reviewed-by: Gabe Black <gabeblack@google.com>