Create and use predefined messages for datatypes which match the ones Accellera uses. Change-Id: I92dd52f62462b864264217bb81f3ff1dcec020bf Reviewed-on: https://gem5-review.googlesource.com/c/13331 Reviewed-by: Gabe Black <gabeblack@google.com> Maintainer: Gabe Black <gabeblack@google.com>