docs: outline some guidelines for security classification

Beyond the overall virt/non-virt use case classification, there are
a number of scenarios which we have decided will not be treated as
security issues. Start to document some of these to give consistency
in our treatment of incoming disclosures.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Mauro Matteo Cascella <mcascell@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>