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>

Cargo.lock

@@ -1015,6 +1015,7 @@ version = "0.1.0"
 dependencies = [
  "bitflags 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "libc 0.2.65 (registry+https://github.com/rust-lang/crates.io-index)",
+ "log 0.4.8 (registry+https://github.com/rust-lang/crates.io-index)",
  "vm-memory 0.1.0 (git+https://github.com/rust-vmm/vm-memory)",
 ]
 

vhost_user_fs/Cargo.toml

@@ -7,4 +7,5 @@ edition = "2018"
 [dependencies]
 bitflags = "1.1.0"
 libc = "0.2.65"
+log = "0.4.8"
 vm-memory = { git = "https://github.com/rust-vmm/vm-memory" }

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;