kvm_config: add CONFIG_S390_GUEST

make kvmconfig currently does not select CONFIG_S390_GUEST. Since
the virtio-ccw transport depends on CONFIG_S390_GUEST, we want
to add CONFIG_S390_GUEST to kvmconfig.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>