block, pci, rate_limiter, vm-allocator: drop legacy numeric constants

Signed-off-by: Wei Liu <liuwe@microsoft.com>

pci/src/vfio_user.rs

@@ -11,7 +11,6 @@ use std::any::Any;
 use std::os::unix::prelude::AsRawFd;
 use std::ptr::null_mut;
 use std::sync::{Arc, Barrier, Mutex};
-use std::u32;
 use thiserror::Error;
 use vfio_bindings::bindings::vfio::*;
 use vfio_ioctls::VfioIrq;