cargo: Clean up serde dependencies

There is no need to include serde_derive separately,
as it can be specified as serde feature instead.

Signed-off-by: Maksym Pavlenko <pavlenko.maksym@gmail.com>

vmm/src/device_manager.rs

@@ -65,6 +65,7 @@ use pci::{
     VfioUserPciDevice, VfioUserPciDeviceError,
 };
 use seccompiler::SeccompAction;
+use serde::{Deserialize, Serialize};
 use std::collections::{BTreeSet, HashMap};
 use std::convert::TryInto;
 use std::fs::{read_link, File, OpenOptions};