target-i386: Update VMStateDescription to X86CPU

Expose vmstate_cpu as vmstate_x86_cpu and hook it up to CPUClass::vmsd.
Adapt opaques and VMState fields to X86CPU. Drop cpu_{save,load}().

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