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>

Cargo.toml

@@ -79,6 +79,7 @@ members = [
     "performance-metrics",
     "qcow",
     "rate_limiter",
+    "serial_buffer",
     "test_infra",
     "vfio_user",
     "vhdx",