fix nfs mount fail with FreeBSD9.1 (synced_folder).

following error occurs in mount of nfs.

```
[default] Mounting NFS shared folders...
The following SSH command responded with a non-zero exit status.
Vagrant assumes that this means the command failed!

mount '10.0.0.1:/Users/foo/bar' '/vagrant'

Stdout from the command:



Stderr from the command:

mount: 10.0.0.1:/Users/foo/bar: No such file or directory
```