storage: rbd: Implement support for passing config file option

To be able to express some use cases of the RBD backing with libvirt, we
need to be able to specify a config file for the RBD client to qemu as
that is one of the commonly used options.

docs/formatdomain.html.in

@@ -1680,6 +1680,7 @@
       <source protocol="rbd" name="image_name2">
         <host name="hostname" port="7000"/>
         <snapshot name="snapname"/>
+        <config file="/path/to/file"/>
       </source>
       <target dev="hdc" bus="ide"/>
       <auth username='myuser'>
@@ -2023,6 +2024,14 @@
             source for storage protocols.
             Supported for 'rbd' <span class="since">since 1.2.11 (QEMU only).</span>
           </dd>
+          <dt><code>config</code></dt>
+          <dd>
+            The <code>file</code> attribute for the <code>config</code> element
+            provides a fully qualified path to a configuration file to be
+            provided as a parameter to the client of a networked storage
+            protocol. Supported for 'rbd' <span class="since">since 1.2.11
+            (QEMU only).</span>
+          </dd>
         </dl>
 
         <p>