Make Qemu be the vhost-user server

[jerome-jutteau]

In order to reconnect a VM machine to a (new) Butterfly, qemu needs to be the "server" in vhost-user and Butterfly the "client". Check packetgraph's issue.

1. Adapt packetgraph to manage vhost as a client
2. Make qemu create it's vhost file and add tests in packetgraph to check communication is ok
3. Add deconnection/reconnection tests in packetgraph
4. Add a "vhost client" option in Butterfly during vnic creation and choose a path to vhost-user socket.
5. Adapt graph.cc
6. Add tests to try (re)connections

[jerome-jutteau]

Client's mode in DPDK don't seems to work and vhost PMD client option has not been enabled yet (17.08). Forcing option produce some bogus behaviors (like queues cannot empty or such).
Reporting this feature once DPDK support for client mode will be ready