xmlapi: Rework xmlns setting slightly

So the xmlns is not always set on the top element. We need this for
custom <metadata> support

tests/cli-test-xml/compare/virt-install-many-devices.xml

@@ -1,4 +1,4 @@
-<domain xmlns:qemu="http://libvirt.org/schemas/domain/qemu/1.0" type="kvm">
+<domain type="kvm">
   <name>foobar</name>
   <uuid>00000000-1111-2222-3333-444444444444</uuid>
   <memory>65536</memory>
@@ -400,7 +400,7 @@
       <address iobase="507" type="isa"/>
     </panic>
   </devices>
-  <qemu:commandline>
+  <qemu:commandline xmlns:qemu="http://libvirt.org/schemas/domain/qemu/1.0">
     <qemu:arg value="-display"/>
     <qemu:arg value="gtk,gl=on"/>
     <qemu:arg value="-device"/>

tests/cli-test-xml/compare/virt-xml-edit-simple-qemu-commandline.xml

@@ -1,13 +1,7 @@
--<domain type="test">
-+<domain xmlns:qemu="http://libvirt.org/schemas/domain/qemu/1.0" type="test">
-   <name>test-for-virtxml</name>
-   <uuid>12345678-12f4-1234-1234-123456789012</uuid>
-   <description>Test VM for virtxml cli tests
-@@
      </panic>
    </devices>
    <seclabel type="dynamic" model="selinux" relabel="yes"/>
-+  <qemu:commandline>
++  <qemu:commandline xmlns:qemu="http://libvirt.org/schemas/domain/qemu/1.0">
 +    <qemu:arg value="-foo"/>
 +    <qemu:arg value="bar,baz=wib wob"/>
 +  </qemu:commandline>

virtinst/xmlapi.py

@@ -97,7 +97,7 @@ class _XMLBase(object):
         raise NotImplementedError()
     def _node_set_property(self, node, propname, setval):
         raise NotImplementedError()
-    def _node_new(self, xpathseg):
+    def _node_new(self, xpathseg, parentnode):
         raise NotImplementedError()
     def _node_add_child(self, parentxpath, parentnode, newnode):
         raise NotImplementedError()
@@ -212,7 +212,7 @@ class _XMLBase(object):
                 parentnode = tmpnode
                 continue
 
-            newnode = self._node_new(xpathseg)
+            newnode = self._node_new(xpathseg, parentnode)
             self._node_add_child(oldxpath, parentnode, newnode)
             parentnode = newnode
 
@@ -307,17 +307,22 @@ class _Libxml2API(_XMLBase):
         else:
             node.setProp(propname, util.xml_escape(setval))
 
-    def _node_new(self, xpathseg):
+    def _node_new(self, xpathseg, parentnode):
         newnode = libxml2.newNode(xpathseg.nodename)
         if not xpathseg.nsname:
             return newnode
 
-        ctxnode = self._ctx.contextNode()
-        for ns in util.listify(ctxnode.nsDefs()):
-            if ns.name == xpathseg.nsname:
-                break
-        else:
-            ns = ctxnode.newNs(
+        def _find_parent_ns():
+            parent = parentnode
+            while parent:
+                for ns in util.listify(parent.nsDefs()):
+                    if ns.name == xpathseg.nsname:
+                        return ns
+                parent = parent.get_parent()
+
+        ns = _find_parent_ns()
+        if not ns:
+            ns = newnode.newNs(
                     self.NAMESPACES[xpathseg.nsname], xpathseg.nsname)
         newnode.setNs(ns)
         return newnode