storage: Introduce virStorageBackendDeviceIsEmpty
    
    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: John Ferlan <jferlan@redhat.com>