sched: get rid of the per domain vCPU list in Credit2

sched: get rid of the per domain vCPU list in Credit2

As, curently, there is no reason for bothering having
it and keeping it updated.

In fact, it is only used for dumping and changing
vCPUs parameters, but that can be achieved easily with
for_each_vcpu.

While there, improve alignment of comments, ad
add a const qualifier to a pointer, making things
more consistent with what happens everywhere else
in the source file.

This also allows us to kill one of the remaining
FIXMEs in the code, which is always good.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>