Vbox shutdown issue

Fixes #512  by adding a small pause before starting the export.

The problem I was seeing was that during a packer build for Virtualbox, it would get to the export phase, attempt to delete the forwarding ports and fail with the following error:
```
==> virtualbox: Preparing to export machine...
    virtualbox: Deleting forwarded port mapping for SSH (host port 3499)
==> virtualbox: Error deleting port forwarding rule: VBoxManage error: VBoxManage: error: The machine 'packer-virtualbox' is already locked for a session (or being unlocked)
==> virtualbox: VBoxManage: error: Details: code VBOX_E_INVALID_OBJECT_STATE (0x80bb0007), component Machine, interface IMachine, callee nsISupports
==> virtualbox: VBoxManage: error: Context: "LockMachine(a->session, LockType_Write)" at line 425 of file VBoxManageModifyVM.cpp
==> virtualbox: Error unregistering ISO: VBoxManage error: VBoxManage: error: Failed to get a console object from the direct session (Unknown Status 0x80BB0002)
==> virtualbox: VBoxManage: error: Details: code VBOX_E_VM_ERROR (0x80bb0003), component Machine, interface IMachine, callee nsISupports
==> virtualbox: VBoxManage: error: Context: "LockMachine(a->session, LockType_Shared)" at line 314 of file VBoxManageStorageController.cpp
==> virtualbox: Unregistering and deleting virtual machine...
==> virtualbox: Deleting output directory...
```

Sometimes however the build would proceed with no timing issues.

Using this as a conversation starter since I am not sure where the real problem lies, but this certainly fixes my problem of intermittent failures to delete the forwarding ports.