cpu: Make accesses to comInstEventQueue indirect through methods.

cpu: Make accesses to comInstEventQueue indirect through methods.

This lets us move the event queue itself around, or change how those
services are provided.

Change-Id: Ie36665b353cf9788968f253cf281a854a6eff4f4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22107
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>