vmm: device_manager: Make PtyPair implement Clone

The clone method for PtyPair should have been an impl of the Clone
trait but the method ended up not being used. Future work will make
use of the trait however so correct the missing trait implementation.

Signed-off-by: William Douglas <william.douglas@intel.com>

vmm/src/device_manager.rs

@@ -766,7 +766,7 @@ pub struct PtyPair {
     pub path: PathBuf,
 }
 
-impl PtyPair {
+impl Clone for PtyPair {
     fn clone(&self) -> Self {
         PtyPair {
             main: self.main.try_clone().unwrap(),