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/Cargo.toml

@@ -41,6 +41,7 @@ qcow = { path = "../qcow" }
 seccompiler = "0.2.0"
 serde = { version = "1.0.144", features = ["rc", "derive"] }
 serde_json = "1.0.85"
+serial_buffer = { path = "../serial_buffer" }
 signal-hook = "0.3.14"
 thiserror = "1.0.32"
 uuid = "1.1.2"