target-i386: Move setting defaults out of cpu_x86_parse_featurestr()

No functional change, needed for simplifying conversion to properties.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>