xmlbuilder: Order child props before serializing

We were implicitly depending on dict hash ordering here, which
was causing some different XML output in centos CI

tests/cli-test-xml/compare/virt-install-singleton-config-2.xml

@@ -91,6 +91,7 @@
     <feature policy="disable" name="distest"/>
     <feature policy="forbid" name="foo"/>
     <feature policy="forbid" name="bar"/>
+    <cache mode="emulate" level="3"/>
     <numa>
       <cell id="0" cpus="1,2,3" memory="1024">
         <distances>
@@ -105,7 +106,6 @@
         </distances>
       </cell>
     </numa>
-    <cache mode="emulate" level="3"/>
   </cpu>
   <clock offset="utc">
     <timer name="rtc" tickpolicy="merge"/>
@@ -259,6 +259,7 @@
     <feature policy="disable" name="distest"/>
     <feature policy="forbid" name="foo"/>
     <feature policy="forbid" name="bar"/>
+    <cache mode="emulate" level="3"/>
     <numa>
       <cell id="0" cpus="1,2,3" memory="1024">
         <distances>
@@ -273,7 +274,6 @@
         </distances>
       </cell>
     </numa>
-    <cache mode="emulate" level="3"/>
   </cpu>
   <clock offset="utc">
     <timer name="rtc" tickpolicy="merge"/>

virtinst/xmlbuilder.py

@@ -845,7 +845,7 @@ class XMLBuilder(object):
             elif key in childprops:
                 do_order.insert(0, key)
 
-        for key in list(childprops.keys()):
+        for key in sorted(list(childprops.keys())):
             if key not in do_order:
                 do_order.append(key)