conf: Set defaultFormat if no storage source XML present

While parsing if the storage source is not present, then a defaultFormat
was not set. This could lead to oddities such as seeing "unknown" format
in output for the "logical" pool even though the only format the pool could
support would be "lvm2".

This does "put a label" on other pool defaults as follows:

   File System: FS_AUTO
   Network File System: NETFS_AUTO
   Disk: UNKNOWN

Each of which is the "0" value for their respective pools and thus
would be no "real" change.

src/conf/storage_conf.c

@@ -703,6 +703,9 @@ virStoragePoolDefParseXML(xmlXPathContextPtr ctxt)
         if (virStoragePoolDefParseSource(ctxt, &ret->source, ret->type,
                                          source_node) < 0)
             goto error;
+    } else {
+        if (options->formatFromString)
+            ret->source.format = options->defaultFormat;
     }
 
     ret->name = virXPathString("string(./name)", ctxt);