graph: Backend must have name and description.

* guix/graph.scm (<graph-backend>): Add fields "name" and "description".
(%graphviz-backend): Provide values for name and description.
(export-graph): Ignore name and description when matching backends.
(graph-backend-name, graph-backend-description): New procedures.
* tests/graph.scm (make-recording-backend): Initialize name and
description fields of test graph-backend.