Fix could not capture caption of graphviz node by xref

std domain and LaTeX writer expect figure node for capturing captions,
but graphviz directive returns graphviz node including caption attribute.
So sphinx cannot capture it on resolving xref.

This change makes graphviz directive returns figure and caption nodes in place.
Merged in tk0miya/sphinx (pull request #265)

Fix could not capture caption of graphviz node by xref