cpu: Abstract CPU core type

Add an abstract CPU core type that could be used by machines that want
to define and hotplug CPUs in core granularity.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
               [Integer core property]
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
[dwg: changed property names to 'core-id' and 'nr-threads']
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>