storage: Generate correct parameters for CIFS

https://bugzilla.redhat.com/show_bug.cgi?id=1186969

When generating the path to the dir for a CIFS/Samba driver, the code
would generate a source path for the mount using "%s:%s" while the
mount.cifs expects to see "//%s/%s". So check for the cifsfs and
format the source path appropriately.

Additionally, since there is no means to authenticate, the mount
needs a "-o guest" on the command line in order to anonymously mount
the Samba directory.

docs/formatstorage.html.in

@@ -124,11 +124,14 @@
         <span class="since">Since 0.4.1</span></dd>
       <dt><code>dir</code></dt>
       <dd>Provides the source for pools backed by directories (pool
-        type <code>dir</code>), or optionally to select a subdirectory
+        types <code>dir</code>, <code>netfs</code>, <code>gluster</code>),
+        or optionally to select a subdirectory
         within a pool that resembles a filesystem (pool
         type <code>gluster</code>). May
         only occur once. Contains a single attribute <code>path</code>
-        which is the fully qualified path to the backing directory.
+        which is the fully qualified path to the backing directory or
+        for a <code>netfs</code> pool type using <code>format</code>
+        type "cifs", the path to the Samba share without the leading slash.
         <span class="since">Since 0.4.1</span></dd>
       <dt><code>adapter</code></dt>
       <dd>Provides the source for pools backed by SCSI adapters (pool

docs/storage.html.in

@@ -291,7 +291,8 @@
         the <a href="#StorageBackendGluster">gluster</a> pool.)
       </li>
       <li>
-        <code>cifs</code> - use the SMB (samba) or CIFS file system
+        <code>cifs</code> - use the SMB (samba) or CIFS file system.
+        The mount will use "-o guest" to mount the directory anonymously.
       </li>
     </ul>