vhost: add security model documentation

vhost: add security model documentation

Input validation is not applied consistently in vhost_user.c.  This
suggests that not everyone has the same security model in mind when
working on the code.

Make the security model explicit so that everyone can understand and
follow the same model when modifying the code.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: John McNamara <john.mcnamara@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>