xen/cpupool: switch cpupool list to normal list interface

xen/cpupool: switch cpupool list to normal list interface

Instead of open coding something like a linked list just use the
available functionality from list.h.

The allocation of a new cpupool id is not aware of a possible wrap.
Fix that. We don't, however, consider the case of extremely many pools
(beyond 4 billion) as something which needs explicitly handling right
now. First and foremost there would need to be systems with 4 billion
CPUs to make this many pools a sensible thing to have.

While adding the required new include to private.h sort the includes.

Signed-off-by: From: Juergen Gross <jgross@suse.com>
Reviewed-by: Dario Faggioli <dfaggioli@suse.com>