xmlapi: Move namespace mappings to the relevant classes

So we don't need to update xmlapi.py to account for additions

virtinst/domain/xmlnsqemu.py

@@ -7,6 +7,10 @@
 from ..xmlbuilder import XMLBuilder, XMLChildProperty, XMLProperty
 
 
+XMLBuilder.register_namespace(
+        "qemu", "http://libvirt.org/schemas/domain/qemu/1.0")
+
+
 class _XMLNSQemuArg(XMLBuilder):
     XML_NAME = "qemu:arg"
 

virtinst/xmlapi.py

@@ -76,9 +76,10 @@ class _XPath(object):
 
 
 class _XMLBase(object):
-    NAMESPACES = {
-        "qemu": "http://libvirt.org/schemas/domain/qemu/1.0",
-    }
+    NAMESPACES = {}
+    @classmethod
+    def register_namespace(cls, nsname, uri):
+        cls.NAMESPACES[nsname] = uri
 
     def copy_api(self):
         raise NotImplementedError()

virtinst/xmlbuilder.py

@@ -419,6 +419,11 @@ class XMLBuilder(object):
     # https://bugzilla.redhat.com/show_bug.cgi?id=1184131
     _XML_SANITIZE = False
 
+    @staticmethod
+    def register_namespace(nsname, uri):
+        XMLAPI.register_namespace(nsname, uri)
+
+
     def __init__(self, conn, parsexml=None,
                  parentxmlstate=None, relative_object_xpath=None):
         """