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_tree.rs

@@ -4,6 +4,7 @@
 
 use crate::device_manager::PciDeviceHandle;
 use pci::PciBdf;
+use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
 use std::sync::{Arc, Mutex};
 use vm_device::Resource;