Live migration is broken for NFS shared storage

One of the new checks, introduced in
I2755c59b4db736151000dae351fd776d3c15ca39
is missing a check against filebased shared storage,
leading to live migration being broken.

Closes-Bug: #1346385
Change-Id: I096ecd6eadc3b13ef918ca1e6c98acdd01556c0d