xen/cpupool: limit number of cpupools

xen/cpupool: limit number of cpupools

Today the number of cpupools in a system is unlimited. This can lead to
multiple problems (e.g. duplicate cpupool-id or scalability issues).

Limit the number of cpupools to twice the number of maximum possible
cpus, allowing to have one cpupool per physical cpu plus some spare
cpupools for special means (there are already existing use cases for
such spare cpupools).

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>