x86: Use a std::function to handle MSI completion.

x86: Use a std::function to handle MSI completion.

This removes the recvResponse callback from the IntMasterPort, and
makes it easier to handle the default case where we just need to clean
up the Packet.

Change-Id: I8bcbfee0aaf68b12310d773f925c399fc87ea65d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20828
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>