target-i386: Move TCG initialization to realize time

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()).

Move TCG initialization to realize time so it won't be called when just
doing object_new() on a X86CPU subclass.

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