Documentation: s390-diag.rst: Make diag500 a generic KVM hypercall

Documentation: s390-diag.rst: Make diag500 a generic KVM hypercall

Let's make it a generic KVM hypercall, allowing other subfunctions to
be more independent of virtio.

While at it, document that unsupported/unimplemented subfunctions result
in a SPECIFICATION exception.

This is a preparation for documenting a new subfunction.

Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Tested-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Link: https://lore.kernel.org/r/20241025141453.1210600-2-david@redhat.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>