s390x/s390-virtio-hcall: remove hypercall registration mechanism

s390x/s390-virtio-hcall: remove hypercall registration mechanism

Nowadays, we only have a single machine type in QEMU, everything is based
on virtio-ccw and the traditional virtio machine does no longer exist. No
need to dynamically register diag500 handlers. Move the two existing
handlers into s390-virtio-hcall.c.

Message-ID: <20241219144115.2820241-3-david@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>