file: Fix file command on /dev/VG/LV paths (RHBZ#582484).

Previous commit 4df593496e116dfb635731c058b7627e81fc179c broke the
"file" command on logical volume paths, since these are symbolic
links.  We *should* follow these (only).

This inadvertantly broke virt-inspector too, which indicates that
we need more regression testing in this area.  Since carrying whole
Fedora images around could make the distribution even larger than
now, I'm not sure at the moment how to do this.

Thanks to Matt Booth for diagnosing this bug.