s390x/ipl: Add type and length checks for IplParameterBlock values

We can check for valid type and lengths of the IplParameterBlock fields
when receiving the struct from the guest.

Length of the IplParameterBlock can be less than 4K. To play safe we can
read and write only required amount of data.

Signed-off-by: Alexander Yarygin <yarygin@linux.vnet.ibm.com>
Reviewed-by: David Hildenband <dahi@linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
