guest: Fix adding a parsed device to a built guest

tests/xmlparse-xml/add-devices-out.xml

@@ -38,6 +38,7 @@
     </interface>
     <graphics type="sdl" display=":3.4" xauth="/tmp/.Xauthority"/>
     <console type="pty"/>
+    <sound model="pcspk"/>
     <interface type="network">
       <source network="default"/>
       <mac address="1A:2A:3A:4A:5A:6A"/>

tests/xmlparse.py

@@ -756,6 +756,9 @@ class XMLParseTest(unittest.TestCase):
         guest.remove_device(adddev)
         guest.add_device(adddev)
 
+        guest.add_device(virtinst.VirtualAudio(conn,
+            parsexml="""<sound model='pcspk'/>"""))
+
         self._alter_compare(guest.get_xml_config(), outfile)
 
     def testChangeKVMMedia(self):

virtinst/guest.py

@@ -212,8 +212,8 @@ class Guest(XMLBuilder):
         @param set_defaults: Whether to set defaults for the device
         """
         self._track_device(dev)
-        self._add_child(dev)
         self._recalculate_device_xpaths()
+        self._add_child(dev)
 
     def _track_device(self, dev):
         self._devices.append(dev)

virtinst/xmlbuilder.py

@@ -795,9 +795,19 @@ class XMLBuilder(object):
         specified path
         """
         if not dev._xmlstate.is_build:
-            newnode = libxml2.parseDoc(dev.get_xml_config()).children
-            _build_xpath_node(self._xmlstate.xml_ctx,
-                              dev.get_root_xpath(), newnode)
+            if not dev.get_root_xpath():
+                raise RuntimeError("programming error: must set device "
+                                   "root xpath before add_child")
+            orig_xpath = dev.get_root_xpath()
+            dev.set_root_xpath(None)
+            xml = dev.get_xml_config()
+            dev.set_root_xpath(orig_xpath)
+
+            use_xpath = orig_xpath.rsplit("/", 1)[0]
+            newnode = libxml2.parseDoc(xml).children
+            _build_xpath_node(self._xmlstate.xml_ctx, use_xpath, newnode)
+            dev.set_root_xpath(orig_xpath)
+
         dev._xmlstate._parse(None, self._xmlstate.xml_node)
 
     def _remove_child(self, dev):