Rename virStorageBackendFileSystemProbe and to virStorageBackendBLKIDFindFS
and move to the more common storage_backend module.

Create a shim virStorageBackendDeviceIsEmpty which will make the call
to the virStorageBackendBLKIDFindFS and check the return value.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>