The IplParameterBlock struct currently has only 200 bytes filled, but it
can be up to 4K.

This patch converts the struct to union with a fully populated struct
inside it and second struct with old values.

For compatibility reasons we disable migration of the extended iplb
field for pre-2.7 machines. Also a guest still can read/write only the
first 200 bytes of IPLB for now.
Signed-off-by: NAlexander Yarygin <yarygin@linux.vnet.ibm.com>
Reviewed-by: NDavid Hildenbrand <dahi@linux.vnet.ibm.com>
Acked-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>