VBOX XML files are not locked by VBOX Manager instances

[Z6543]

We have lost snapshots recently because the .vbox XML files are not locked by the Virtualbox Manager.

Steps to reproduce:

1. Create a new VM in User1 Windows account.
2. Don't quit from the Virtualbox Manager GUI.
3. Switch to Windows user account User2.
4. Open the same VM in the Virtualbox Manager.
5. Create a new snapshot (Snapshot 1).
6. Close the Virtualbox Manager in User2 account, logoff from User2.
7. Switch back to User1. Snapshot 1 is not listed.
8. Close the Virtualbox Manager.
9. Start the Virtualbox Manager. Snapshot 1 is not listed.

Expected result: When the User2 opens the VM in the Virtualbox Manager, it should raise an error that this VM is locked by another user.

These errors never happen intentionally, so it would be great if the Virtualbox Manager could lock the vbox files.

Sometimes we can recover the snapshots by manually recreating the snapshot entries in the vbox XML file, but this is not optimal.

Thank you for your help