vm-migration: Make snapshot() mutable

There will be some cases where the implementation of the snapshot()
function from the Snapshottable trait will require to modify some
internal data, therefore we make this possible by updating the trait
definition with snapshot(&mut self).

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

vm-migration/src/lib.rs

@@ -111,7 +111,7 @@ pub trait Snapshottable: Pausable {
     }
 
     /// Take a component snapshot.
-    fn snapshot(&self) -> std::result::Result<Snapshot, MigratableError> {
+    fn snapshot(&mut self) -> std::result::Result<Snapshot, MigratableError> {
         Ok(Snapshot::new(""))
     }