title: VM in virtualbox is already locked for a session (or being unlocked)
body: <p>My VM in virtualbox can not start due to this error, I don't want to destroy it and reinstall it again, anyway to recover it ?</p>
  
  <p>There was an error while executing <code>VBoxManage</code>, a CLI used by Vagrant
  for controlling VirtualBox. The command and stderr is shown below.</p>
  
  <pre><code>Command: ["modifyvm", "319fcce3-e8ff-4b6f-a641-3aee1df6543f", "--natpf1", "delete", "ssh"]
  
  Stderr: VBoxManage: error: The machine 'centos64_c6402_1454036461345_59755' is already locked for a session (or being unlocked)
  VBoxManage: error: Details: code VBOX_E_INVALID_OBJECT_STATE (0x80bb0007), component MachineWrap, interface IMachine, callee nsISupports
  VBoxManage: error: Context: "LockMachine(a-&gt;session, LockType_Write)" at line 493 of file VBoxManageModifyVM.cpp
  </code></pre>