vmm: vm: Implement the Snapshottable trait

By aggregating snapshots from the CpuManager, the MemoryManager and the
DeviceManager, Vm implements the snapshot() function from the
Snapshottable trait.
And by restoring snapshots from the CpuManager, the MemoryManager and
the DeviceManager, Vm implements the restore() function from the
Snapshottable trait.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Yi Sun <yi.y.sun@linux.intel.com>

vmm/src/lib.rs

@@ -679,3 +679,7 @@ impl Vmm {
         Ok(())
     }
 }
+
+const CPU_MANAGER_SNAPSHOT_ID: &str = "cpu-manager";
+const MEMORY_MANAGER_SNAPSHOT_ID: &str = "memory-manager";
+const DEVICE_MANAGER_SNAPSHOT_ID: &str = "device-manager";