storage: Fix logical pool fmt type

According to our documentation logical pool supports formats 'auto' and
'lvm2'. However, in storage_conf.c we previously defined storage pool
formats: unknown, lvm2. Due to backward compatibility reasons
we must continue refer to pool format type 'unknown' instead of 'auto'.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1123767

docs/schemas/storagepool.rng

@@ -465,7 +465,7 @@
       <element name='format'>
         <attribute name='type'>
           <choice>
-            <value>auto</value>
+            <value>unknown</value> <!-- back-compat requires keeping 'unknown' not 'auto' -->
             <value>lvm2</value>
           </choice>
         </attribute>

docs/storage.html.in

@@ -328,14 +328,10 @@
 
     <h3>Valid pool format types</h3>
     <p>
-      The logical volume pool supports the following formats:
+      The logical volume pool supports only the <code>lvm2</code> format,
+      although not supplying a format value will result in automatic
+      selection of the<code>lvm2</code> format.
     </p>
-    <ul>
-      <li><code>auto</code> - automatically determine format</li>
-      <li>
-        <code>lvm2</code>
-      </li>
-    </ul>
 
     <h3>Valid volume format types</h3>
     <p>