Add 'nbd' as a valid filesystem driver type

The <filesystem> element can now accept a <driver type='nbd'/>
as an alternative to 'loop'. The benefit of NBD is support
for non-raw disk image formats.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

docs/formatdomain.html.in

@@ -1992,8 +1992,9 @@
             the <code>type</code> attribute selects the primary
             backend driver name, while the <code>format</code>
             attribute provides the format type. For example, LXC
-            supports a type of "loop", with a format of "raw". QEMU
-            supports a type of "path" or "handle", but no formats.
+            supports a type of "loop", with a format of "raw" or
+            "nbd" with any format. QEMU supports a type of "path"
+            or "handle", but no formats.
           </li>
         </ul>
       </dd>

docs/schemas/domaincommon.rng

@@ -1675,6 +1675,7 @@
             <value>path</value>
             <value>handle</value>
             <value>loop</value>
+            <value>nbd</value>
           </choice>
         </attribute>
       </optional>