Create and use predefined messages for channels which match the ones Accellera uses. Change-Id: I179214838bbd83604e50225926cdc6b5b1b16923 Reviewed-on: https://gem5-review.googlesource.com/c/13330 Reviewed-by: Gabe Black <gabeblack@google.com> Maintainer: Gabe Black <gabeblack@google.com>