Fix #4658. Bad NFS exports file on OS X & BSD hosts.

For FreeBSD guests, Virtualbox can sometimes report the private network
interface IP address as "0.0.0.0".  This will cause an invalid NFS
exports file to be generated for FreeBSD and OS X hosts.

Fixed by not allowing Virtualbox to report a guest IP address of
"0.0.0.0".
Merge pull request #4671 from jramnani/fix/master/nfs_exports_file_for_freebsd_guests

Fix #4658. Bad NFS exports file on OS X & BSD hosts.