nodedev: Resolve Relax-NG validity error

Commit id 'c4a4603de' added an output <path> to the nodedev xml, but
did not update the schema.

This resulted in the failure of the 'virt-xml-validate' on a file
generated by 'virsh nodedev-dumpxml pci_0000_00_00_0' (for example).

This was found/seen by running autotest on my host.

docs/schemas/nodedev.rng

@@ -12,6 +12,9 @@
       <!-- The name of the network, used to refer to it through the API
          and in virsh -->
       <element name="name"><text/></element>
+      <optional>
+        <element name="path"><text/></element>
+      </optional>
       <optional>
         <element name="parent"><text/></element>
       </optional>