misc: vmm: drop extern crate, use modern rust

This commit is part of a series of similar commits.

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
On-behalf-of: SAP philipp.schuster@sap.com

vmm/src/serial_manager.rs

@@ -19,6 +19,7 @@ use devices::legacy::Pl011;
 #[cfg(any(target_arch = "x86_64", target_arch = "riscv64"))]
 use devices::legacy::Serial;
 use libc::EFD_NONBLOCK;
+use log::{error, info, warn};
 use serial_buffer::SerialBuffer;
 use thiserror::Error;
 use vmm_sys_util::eventfd::EventFd;