Add a get_root_bdm utility function

Getting the root block device out of a list of block devices is a common
idiom and we use it in several places in our code base. This patch
factors it out in a handy utilty method and adds tests for it.

Change-Id: I4a2cb3c689fed9fbc0bc51ddba095e1269cacb2d
blueprint: clean-up-legacy-block-device-mapping

nova/block_device.py

@@ -388,6 +388,13 @@ def new_format_is_ephemeral(bdm):
     return False
 
 
+def get_root_bdm(bdms):
+    try:
+        return (bdm for bdm in bdms if bdm.get('boot_index', -1) == 0).next()
+    except StopIteration:
+        return None
+
+
 def mappings_prepend_dev(mappings):
     """Prepend '/dev/' to 'device' entry of swap/ephemeral virtual type."""
     for m in mappings: