vhost_user_fs: Implement FileSystem trait for Passthrough

Add a "passthrough" file system implementation that just forwards its
requests to the appropriate system call.

This code has been ported over from crosvm commit
961461350c0b6824e5f20655031bf6c6bf6b7c30.

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

vhost_user_fs/src/lib.rs

@@ -2,6 +2,10 @@
 //
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
+#[macro_use]
+extern crate log;
+
 pub mod filesystem;
 pub mod fuse;
 pub mod multikey;
+pub mod passthrough;