mem: Clean up the xbars a little.

mem: Clean up the xbars a little.

Get rid of comments which just restate the code, get rid of redundant
"virtual" keywords, add "override"s, fix style, and get rid of
xbar::init which was empty and hiding the parent class init.

Change-Id: I8ce20abee340baa88084d142f2fb8c633ee54ba9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17592
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>