s390x/kvm: enable guarded storage

Introduce guarded storage support for KVM guests on s390.
We need to enable the capability, extend machine check validity,
sigp store-additional-status-at-address, and migration.

The feature is fenced for older machine type versions.

Signed-off-by: Fan Zhang <zhangfan@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
