vmm: Add support for spawning vhost-user-net backend

If no socket is supplied when enabling "vhost_user=true" on "--net"
follow the "exe" path in the /proc entry for this process and launch the
network backend (via the vmm_path field.)

Currently this only supports creating a new tap interface as the network
backend also only supports that.

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

vmm/src/config.rs

@@ -587,11 +587,6 @@ impl NetConfig {
             vhost_socket = Some(vhost_socket_str.to_owned());
         }
 
-        // For now we require a socket if vhost-user is turned on
-        if vhost_user && vhost_socket.is_none() {
-            return Err(Error::ParseNetVhostSocketRequired);
-        }
-
         Ok(NetConfig {
             tap,
             ip,