s390x/s390-virtio-hcall: prepare for more diag500 hypercalls

s390x/s390-virtio-hcall: prepare for more diag500 hypercalls

Let's generalize, abstracting the virtio bits. diag500 is now a generic
hypercall to handle QEMU/KVM specific things. Explicitly specify all
already defined subcodes, including legacy ones (so we know what we can
use for new hypercalls).

Move the PGM_SPECIFICATION injection into the renamed function
handle_diag_500(), so we can turn it into a void function.

We'll rename the files separately, so git properly detects the rename.

Message-ID: <20241219144115.2820241-4-david@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>