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/Cargo.toml

@@ -37,6 +37,7 @@ vm-device = { path = "../vm-device" }
 vm-virtio = { path = "../vm-virtio" }
 vmm-sys-util = ">=0.3.1"
 signal-hook = "0.1.13"
+tempfile = "3.1.0"
 
 [dependencies.linux-loader]
 git = "https://github.com/rust-vmm/linux-loader"