dev: Refactor the EtherTapStub to make room for using tap.

dev: Refactor the EtherTapStub to make room for using tap.

A lot of the implementation of EtherTapStub can be shared with a version
which uses a tap device directly. This change factors out those parts to
accommodate that.

Change-Id: I9c2e31f1be139ca73859a83f05457cef90101006
Reviewed-on: https://gem5-review.googlesource.com/3645
Reviewed-by: Nathan Binkert <nate@binkert.org>
Maintainer: Nathan Binkert <nate@binkert.org>