tests: qgraph API for the qtest driver framework

tests: qgraph API for the qtest driver framework

Add qgraph API that allows to add/remove nodes and edges from the graph,
implementation of Depth First Search to discover the paths and basic unit
test to check correctness of the API.
Included also a main executable that takes care of starting the framework,
create the nodes, set the available drivers/machines, discover the path and
run tests.

graph.h provides the public API to manage the graph nodes/edges
graph_extra.h provides a more private API used successively by the gtest integration part
qos-test.c provides the main executable

Signed-off-by: Emanuele Giuseppe Esposito <e.emanuelegiuseppe@gmail.com>
[Paolo's changes compared to the Google Summer of Code submission:
 * added subprocess to test options
 * refactored object creation to support live migration tests
 * removed driver .before callback (unused)
 * removed test .after callbacks (replaced by GTest destruction queue)]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>