Add host_set private structure piix_host_priv.  Currently the only
field is ->map which used to be stored directly at
host_set->private_data.  This change allows more host_set private
fields to be added.
Signed-off-by: NTejun Heo <htejun@gmail.com>
Signed-off-by: NJeff Garzik <jeff@garzik.org>