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

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

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

vmm/src/config.rs

@@ -45,8 +45,6 @@ pub enum Error {
     ParseDiskQueueSizeParam(std::num::ParseIntError),
     /// Failed to parse vhost parameters
     ParseDiskVhostParam(std::str::ParseBoolError),
-    /// Need a vhost socket
-    ParseDiskVhostSocketRequired,
     /// Failed parsing disk wce parameter.
     ParseDiskWceParam(std::str::ParseBoolError),
     /// Failed parsing random number generator parameters.
@@ -447,11 +445,6 @@ impl DiskConfig {
             wce = wce_str.parse().map_err(Error::ParseDiskWceParam)?;
         }
 
-        // For now we require a socket if vhost-user is turned on
-        if vhost_user && vhost_socket.is_none() {
-            return Err(Error::ParseDiskVhostSocketRequired);
-        }
-
         Ok(DiskConfig {
             path: PathBuf::from(path_str),
             readonly: parse_on_off(readonly_str)?,