vmm: Move Vm to the new restore design

Now the entire codebase has been moved to the new restore design, we can
complete the work by creating a dedicated restore() function for the Vm
object and get rid of the method restore() from the Snapshottable trait.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vm-migration/src/lib.rs

@@ -264,11 +264,6 @@ pub trait Snapshottable: Pausable {
     fn snapshot(&mut self) -> std::result::Result<Snapshot, MigratableError> {
         Ok(Snapshot::new(""))
     }
-
-    /// Restore a component from its snapshot.
-    fn restore(&mut self, _snapshot: Snapshot) -> std::result::Result<(), MigratableError> {
-        Ok(())
-    }
 }
 
 /// A transportable component can be sent or receive to a specific URL.