KVM: s390: Fix kvm on IBM System z10

The z10 system supports large pages, kvm-s390 doesnt.
Make sure that we dont advertise large pages to avoid the guest crashing as
soon as the guest kernel activates DAT.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>