vhost_user_net: Use config::OptionParser to simplify net backend parsing

Switch to using the recently added OptionParser in the code that parses
the network backend.

Whilst doing this also update the net-backend syntax to use "sock"
rather than socket.

Fixes: #1092
Partially fixes: #1091

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vmm/src/device_manager.rs

@@ -1392,7 +1392,7 @@ impl DeviceManager {
             .args(&[
                 "--net-backend",
                 &format!(
-                    "ip={},mask={},sock={},num_queues={},queue_size={}",
+                    "ip={},mask={},socket={},num_queues={},queue_size={}",
                     net_cfg.ip, net_cfg.mask, &sock, net_cfg.num_queues, net_cfg.queue_size
                 ),
             ])