xen/cpupool: Create different cpupools at boot time

xen/cpupool: Create different cpupools at boot time

Introduce a way to create different cpupools at boot time, this is
particularly useful on ARM big.LITTLE system where there might be the
need to have different cpupools for each type of core, but also
systems using NUMA can have different cpu pools for each node.

The feature on arm relies on a specification of the cpupools from the
device tree to build pools and assign cpus to them.

ACPI is not supported for this feature.

With this patch, cpupool0 can now have less cpus than the number of
online ones, so update the default case for opt_dom0_max_vcpus.

Documentation is created to explain the feature.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>