exec: split cpu_exec_init()

Put in cpu_exec_initfn() what initializes the CPU,
and leave in cpu_exec_init() what adds it to the environment.

As cpu_exec_initfn() is called by all XX_cpu_initfn(), call it
directly in cpu_common_initfn().
cpu_exec_init() is now a realize function, it will be renamed
to cpu_exec_realizefn() and moved to the XX_cpu_realizefn()
function in a following patch.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>