core_id is not needed by linux-user, as the core_id a.k.a. CPU address
is only accessible from kernel space.

Therefore, drop next_core_id and make cpu_index get autoassigned again
for linux-user.

While at it, shield core_id and cpuid completely from linux-user. cpuid
can also only be queried from kernel space.
Suggested-by: NIgor Mammedov <imammedo@redhat.com>
Reviewed-by: NIgor Mammedov <imammedo@redhat.com>
Signed-off-by: NDavid Hildenbrand <david@redhat.com>
Message-Id: <20170928134609.16985-5-david@redhat.com>
Signed-off-by: NCornelia Huck <cohuck@redhat.com>