exec: move cpu_exec_init() calls to realize functions

exec: move cpu_exec_init() calls to realize functions

Modify all CPUs to call it from XXX_cpu_realizefn() function.

Remove all the cannot_destroy_with_object_finalize_yet as
unsafe references have been moved to cpu_exec_realizefn().
(tested with QOM command provided by commit 4c315c27)

for arm:

Setting of cpu->mp_affinity is moved from arm_cpu_initfn()
to arm_cpu_realizefn() as setting of cpu_index is now done
in cpu_exec_realizefn(). To avoid to overwrite an user defined
value, we set it to an invalid value by default, and update
it in realize function only if the value is still invalid.

Signed-off-by: Laurent Vivier <[email protected]>
Reviewed-by: David Gibson <[email protected]>
Reviewed-by: Igor Mammedov <[email protected]>
Reviewed-by: Eduardo Habkost <[email protected]>
Reviewed-by: Andrew Jones <[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>