s390/kvm: basic implementation of diagnose 308 subcode 6

Linux uses a check for subcode 6 to decide if other subcodes are
available. Provide a minimal implementation for subcode 6, as well
as for subcode 5.

Signed-off-by: Eugene (jno) Dvurechenski <jno@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
[Move code from kvm.c into misc_helper.c]
