sim,python: Flip logic on loopback listeners

sim,python: Flip logic on loopback listeners

People are bitten by allowing external connections to gem5 runs too often
(it happend to me again today). This change flips the logic so the
default is to only allow localhost connections.

Change-Id: If9233f5ca383721017b30b5837a26c5042d925fd
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36175
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-by: Ayaz Akram <yazakram@ucdavis.edu>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>