vmm: Move SerialBuffer to its own crate

We want to be able to reuse the SerialBuffer from the virtio-devices
crate, particularly from the virtio-console implementation. That's why
we move the SerialBuffer definition to its own crate so that it can be
accessed from both vmm and virtio-devices crates, without creating any
cyclic dependency.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vmm/src/serial_manager.rs

@@ -5,12 +5,12 @@
 
 use crate::config::ConsoleOutputMode;
 use crate::device_manager::PtyPair;
-use crate::serial_buffer::SerialBuffer;
 #[cfg(target_arch = "aarch64")]
 use devices::legacy::Pl011;
 #[cfg(target_arch = "x86_64")]
 use devices::legacy::Serial;
 use libc::EFD_NONBLOCK;
+use serial_buffer::SerialBuffer;
 use std::fs::File;
 use std::io::Read;
 use std::os::unix::io::{AsRawFd, FromRawFd};