vmstate: add subsections code

vmstate: add subsections code

This commit adds subsections for each device section.
Subsections is the way to handle information that don't need to be sent
to de destination of a migration because its values are not needed.  It is
the way to handle optional information.  Notice that only the source can
decide if the information is optional or not.  The destination needs to
understand all subsections that it receives to have a sucessful load.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>