s390x/cpumodel: allow zpci features in qemu model

AEN and AIS can be provided unconditionally, ZPCI should be turned on
manually.

With -cpu qemu,zpci=on, the guest kernel can now successfully detect
virtio-pci devices under tcg.

Also fixup the order of the MSA_EXT_{3,4} flags while at it.

Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>