python: Improve SimObject's warning when parent specified twice

python: Improve SimObject's warning when parent specified twice

SimObject outputs a warning when its parent is specified more than
once. The cause is most likely that there is unexpected param
specified in the constructor called in the Python interface.

This commit adds a note about this probable cause of this potential
error to the warning message.

Change-Id: I9b6bf5d5fb0c77bfdad5fde42e88f814e8a4b72b
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38359
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>