pc: move pcms->possible_cpus init out of pc_cpus_init()

possible_cpus could be initialized earlier then cpu objects,
i.e. when -smp is parsed so move init code to possible_cpu_arch_ids()
interface func and do initialization on the first call.

it should help later with making -numa cpu/-smp parsing a machine state
properties.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>