Correct getchildren() usage in list_addresses()

This commit fixes the usage of getchildren in the list_addresses
method. Previously, the method was called on body which was a string.
This commit corrects the behavior and runs the method on an etree
object.

Fixes bug 1132796

Change-Id: Iafb8d51257b7bcbfbcf9141c3d424c601a186a89
diff --git a/tempest/services/compute/xml/servers_client.py b/tempest/services/compute/xml/servers_client.py
index 93630df..955d97e 100644
--- a/tempest/services/compute/xml/servers_client.py
+++ b/tempest/services/compute/xml/servers_client.py
@@ -305,7 +305,8 @@
         resp, body = self.get("servers/%s/ips" % str(server_id), self.headers)
 
         networks = {}
-        for child in etree.fromstring(body.getchildren()):
+        xml_list = etree.fromstring(body)
+        for child in xml_list.getchildren():
             network = self._parse_network(child)
             networks.update(**network)