exec: split cpu_exec_init()

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