Libtopo currently only supports building topologies based on a tree structure. The problem is that not all topologies map naturally to a tree structure.
These changes extend libtopo to support building topologies that are based on directed graph structures.
Specifically, this ticket will cover adding new APIs to libtopo for the following:

- building digraphs

- iterating over graph vertices

- iterating over vertice edges

- finding all paths between an arbitrary pair of vertices

- serializing a graph-based topology to XML

- deserialzing a graph-based topology from XML

See the testing notes in the ticket:
https://www.illumos.org/issues/12330