Cleanup _convert_block_devices

_convert_block_devices was instantiating 2 objects for every 1
returned due to the _is_transformable test. This change simplifies the
code by removing the nested function, and at the same time does the
conversion in a single pass, which is more efficient.

Change-Id: Id6b583a1c08d684fcff073dd0289be8e3919ba0a

nova/virt/block_device.py

@@ -360,17 +360,14 @@ class DriverBlankBlockDevice(DriverVolumeBlockDevice):
 
 
 def _convert_block_devices(device_type, block_device_mapping):
-    def _is_transformable(bdm):
+    devices = []
+    for bdm in block_device_mapping:
         try:
-            device_type(bdm)
+            devices.append(device_type(bdm))
         except _NotTransformable:
-            return False
-        return True
-
-    return [device_type(bdm)
-            for bdm in block_device_mapping
-            if _is_transformable(bdm)]
+            pass
 
+    return devices
 
 convert_swap = functools.partial(_convert_block_devices,
                                  DriverSwapBlockDevice)