mem-garnet: Flexible VCs per Vnet for each router

mem-garnet: Flexible VCs per Vnet for each router

This change allows configuring each router with a certain number
of VCs for each VNET. This is beneficial when dealing with
heterogenous link widths in a system. Configuring VCs
for each router allows one to ensure equal throughput
within the network while avoiding head-of-line blocking.
Changing a router's VCs number can be done in topology files
using the vcs_per_vnet value argument of router.

Change-Id: Icf4f510248128429a1a11f19f9802ee96f340611
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32599
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>