virtio-devices, vmm: vhost: net: Add client mode support

Adding the support for an OVS vhost-user backend to connect as the
vhost-user client. This means we introduce with this patch a new
option to our `--net` parameter. This option is called 'server' in order
to ask the VMM to run as the server for the vhost-user socket.

Fixes #1745

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

virtio-devices/src/vhost_user/mod.rs

@@ -24,10 +24,14 @@ pub use self::vu_common_ctrl::VhostUserConfig;
 
 #[derive(Debug)]
 pub enum Error {
+    /// Failed accepting connection.
+    AcceptConnection(io::Error),
     /// Invalid available address.
     AvailAddress,
     /// Queue number  is not correct
     BadQueueNum,
+    /// Failed binding vhost-user socket.
+    BindSocket(io::Error),
     /// Creating kill eventfd failed.
     CreateKillEventFd(io::Error),
     /// Cloning kill eventfd failed.