vhost-user: define conventions for vhost-user backends

vhost-user: define conventions for vhost-user backends

As discussed during "[PATCH v4 00/29] vhost-user for input & GPU"
review, let's define a common set of backend conventions to help with
management layer implementation, and interoperability.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20190308140454.32437-3-marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>