Always use bdm in instance_block_mapping on Xen

It doesn't seem that Xen will set the 'root_device_name' property on
instances.  This is causing instance_block_mapping to return early
without evaluating the bdm before returning the list of device_names
in use.  I'm not sure why instance_block_mapping does this
(optimization?) but on Xen at least it seems that it should not.

Added a check for Xen compute_driver flag in instance_block_mapping to
"guess" the root_device_name (similarlly to what is done in
compute.utils for swap and ephemeral).

fixes bug #1061944

Change-Id: If5b1a2b7377232c78f0629a3624552ecf6ceb0ee

nova/block_device.py

@@ -17,6 +17,9 @@
 
 import re
 
+from nova import flags
+
+FLAGS = flags.FLAGS
 
 DEFAULT_ROOT_DEV_NAME = '/dev/sda1'
 _DEFAULT_MAPPINGS = {'ami': 'sda1',
@@ -89,8 +92,12 @@ def strip_prefix(device_name):
 
 def instance_block_mapping(instance, bdms):
     root_device_name = instance['root_device_name']
+    # NOTE(clayg): remove this when xenapi is setting default_root_device
     if root_device_name is None:
-        return _DEFAULT_MAPPINGS
+        if FLAGS.compute_driver.endswith('xenapi.XenAPIDriver'):
+            root_device_name = '/dev/xvda'
+        else:
+            return _DEFAULT_MAPPINGS
 
     mappings = {}
     mappings['ami'] = strip_dev(root_device_name)