qom: cpus: split cpu_generic_init() on feature parsing and cpu creation parts

it would allow to reuse feature parsing part in various machines
that have CPU features instead of re-implementing the same feature
parsing each time.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <1505318697-77161-2-git-send-email-imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>