hw/arm/virt: explicitly allocate cpu_index for cpus

Currently cpu_index is implicitly auto assigned during
cpu.realize() time cpu_exec_realizefn()->cpu_list_add().

It happens to match index in possible_cpus so take
control over it and make board initialize cpu_index
to possible_cpus index explicitly. It will at least
document that board is in control of it and when
'-device cpu' support comes it will keep cpu_index
stable regardless of order cpus are created so it won't
break migration.
Within this series it will be used for internal
conversion from storing cpu_index based NUMA node
bitmaps to property based mapping with possible_cpus,
And will allow map cpu_index to a CPU entry in
possible_cpus array.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-Id: <1493816238-33120-5-git-send-email-imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
