misc: Add the --listener-loopback-only command line option.

misc: Add the --listener-loopback-only command line option.

This option invokes the Listener::loopbackOnly() static function which
will make the port listeners bind to the loopback device exclusively and
ignore connections on other devices. That prevents external agents like
port scanners from disrupting simulations with spurious connections.

Change-Id: I46b22165046792a6f970826c109bdbce7db25c84
Reviewed-on: https://gem5-review.googlesource.com/3082
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>