Create an image BDM for every instance

This patch makes every boot create a block device mapping if the
instance was started with an image.

These block devices are not used anywhere at the moment but will be used
in the future as part of a more flexible block device API.

This patch also makes sure they are deleted once the instance is deleted
as well.

Change-Id: Ia89d531be71c460f1f82fcfce34b270639a23061
blueprint: improve-block-device-handling

nova/block_device.py

@@ -166,6 +166,21 @@ def is_safe_for_update(block_device_dict):
                       bdm_db_only_fields)
 
 
+def create_image_bdm(image_ref, boot_index=0):
+    """Create a block device dict based on the image_ref.
+
+    This is useful in the API layer to keep the compatibility
+    with having an image_ref as a field in the instance requests
+    """
+    return BlockDeviceDict(
+        {'source_type': 'image',
+         'image_id': image_ref,
+         'delete_on_termination': True,
+         'boot_index': boot_index,
+         'device_type': 'disk',
+         'destination_type': 'local'})
+
+
 def legacy_mapping(block_device_mapping):
     """Transform a list of block devices of an instance back to the
     legacy data format."""