conf: don't check hyperv spinlock retries if disabled

<hyperv>
  <spinlocks state='off'/>
</hyperv>

results in:
error: XML error: missing HyperV spinlock retry count

Don't require retries when state is off and use virXPathUInt
instead of virXPathString to simplify parsing.

https://bugzilla.redhat.com/show_bug.cgi?id=784836#c19

docs/schemas/domaincommon.rng

@@ -4054,9 +4054,11 @@
         <optional>
           <element name="spinlocks">
             <ref name="hypervtristate"/>
-            <attribute name="retries">
-              <data type="integer"/>
-            </attribute>
+            <optional>
+              <attribute name="retries">
+                <data type="unsignedInt"/>
+              </attribute>
+            </optional>
           </element>
         </optional>
       </interleave>