Starting from saved state skips setUpShares

When trying to  start from saved state, virtualbox.setUpShares() fails while running:
```
VBoxManage guestproperty set boot2docker-vm /VirtualBox/GuestAdd/SharedFolders/MountPrefix /
```

The error message gives a hint: `VBoxManage: error: The machine is not mutable (state is Saved)`

Fixes #292
Wouldn't this be simpler if you moved it down above the existing `startvm` line and used `fallthrough` in the `Poweroff`/`Aborted` block? (http://golang.org/ref/spec#Fallthrough_statements)