storage: don't probe non-files
storage: don't probe non-files

https://bugzilla.redhat.com/show_bug.cgi?id=772088

Backing chains can end on a network protocol, such as nbd:xxx; we
should not attempt to probe the file system in this case.

* src/storage/storage_backend_fs.c (virStorageBackendProbeTarget):
Only probe files.
(cherry picked from commit eac74c1f4752d7311def4b2e4848f5fd218e515c)

mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2