Some objects need to know that we are about to dump stats to perform prepare statistics. This is currently done by registering a callback with the stat system. Expose this callback as a virtual method in Stats::Group to make this pattern more convenient. Change-Id: I5aa475b7d04c288e45f5f413ab7a1907b971dae5 Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com> Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21139 Tested-by: kokoro <noreply+kokoro@google.com> Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>