i386: keep cpu_model field in MachineState uptodate

i386: keep cpu_model field in MachineState uptodate

Update cpu_model in MachineState for i386, so that the field can be used
for cpu hotplug, instead of using a static variable.

This patch is rebased on the latest master.

Signed-off-by: Zhu Guihua <zhugh.fnst@cn.fujitsu.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Acked-by: Andreas Färber <afaerber@suse.de>