vhost_user_fs: Add file traits to handle writing volatile memory

The vm_memory implementation for VolatileSlice is able to read and write
to a source or destination which implements a Read or Write trait.

Unfortunately, this is not enough for this specific use case as we need
to be able to write to a file at a specific offset, which is not
provided by the Read or Write trait.

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

@@ -5,6 +5,7 @@
 #[macro_use]
 extern crate log;
 
+pub mod file_traits;
 pub mod filesystem;
 pub mod fuse;
 pub mod multikey;