fastmodel: Let the EVS set an attribute for getSendFunctional to return.

fastmodel: Let the EVS set an attribute for getSendFunctional to return.

The iris CPU model doesn't necessarily know the best way to send
functional packets (what port? what type is that port?), but only has
a generic sc_module pointer to the EVS and so can't call specialized
methods on it. There also isn't any common base class for EVSes to cast
into in a generic way.

This attribute mechanism lets the EVS set up its own sendFunctional
implementation however it needs to using facilities that are built
into generic sc_objects.

Change-Id: I69bf364908c2a5360bd6ce7d3e49ce67c6f771b0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21046
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>