build: Address issues found by 1.43.0 clippy

These are mostly due to use of "bare use" statements and unnecessary vector
creation.

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

src/bin/vhost_user_fs.rs

@@ -10,7 +10,6 @@ extern crate vhost_user_backend;
 extern crate vm_virtio;
 
 use clap::{App, Arg};
-use epoll;
 use futures::executor::{ThreadPool, ThreadPoolBuilder};
 use libc::EFD_NONBLOCK;
 use log::*;