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/lib.rs

@@ -12,6 +12,7 @@ extern crate serde;
 #[macro_use]
 extern crate serde_derive;
 extern crate serde_json;
+extern crate tempfile;
 extern crate vmm_sys_util;
 
 use crate::api::{ApiError, ApiRequest, ApiResponse, ApiResponsePayload, VmInfo, VmmPingResponse};