target-i386: Call cpu_exec_init() on realize

QOM instance_init functions are not supposed to have any side-effects,
as new objects may be created at any moment for querying property
information (see qmp_device_list_properties()).

Calling cpu_exec_init() also affects QEMU's ability to handle errors
during CPU creation, as some actions done by cpu_exec_init() can't be
reverted.

Move cpu_exec_init() call to realize so a simple object_new() won't
trigger it, and so that it is called after some basic validation of CPU
parameters.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
