systemc: Add the "implementation defined" vport function.

systemc: Add the "implementation defined" vport function.

This function is in an "implementation defined" class body in the spec,
and has a comment next to it which says "(for internal use only)" next
to it, but it is still used directly in one of the tests.

Change-Id: Ib3727c93cc531ddd31a24897291dc7e7c97c2b58
Reviewed-on: https://gem5-review.googlesource.com/11354
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>