hw/s390x: provide loadparm property for the machine

In order to specify the LOADPARM value one may now add ",loadparm=xxx"
parameter to the "-machine s390-ccw-virtio" option.

The property setter will normalize and check the value provided much
like the way the HMC does.

The value is stored, but not used at the moment.

Initial patch from Eugene (jno) Dvurechenski.

Signed-off-by: Eugene (jno) Dvurechenski <jno@linux.vnet.ibm.com>
Signed-off-by: Farhan Ali <alifm@linux.vnet.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
