Call kpartx remove in umount, not cleanup

Similar to I697bfbf042816c5ddf170bde9534cc4f0c7279ff, the order of
things called is "dib-block-device umount" *then* "dib-block-device
cleanup".

Because we're doing the "kpartx -d" here in cleanup, it means that the
loop-device is removed in umount phase from level0/localloop.py, then
afterwards we try and remove the partitions.

Change-Id: I7af3c5cf66afd81a481f454b5207af552ad52a32
TODO: a test case to ensure the ordering

diskimage_builder/block_device/level1/partition.py

@@ -84,5 +84,8 @@ class PartitionNode(NodeBase):
     def create(self):
         self.partitioning.create()
 
+    def umount(self):
+        self.partitioning.umount()
+
     def cleanup(self):
         self.partitioning.cleanup()

diskimage_builder/block_device/level1/partitioning.py

@@ -215,13 +215,15 @@ class Partitioning(PluginBase):
 
         return
 
-    def cleanup(self):
+    def umount(self):
         # remove the partition mappings made for the parent
         # block-device by create() above.  this is called from the
-        # child PartitionNode umount/delete/cleanup.  Thus every
-        # partition calls it, but we only want to do it once and our
-        # gate.
+        # child PartitionNode umount.  Thus every partition calls it,
+        # but we only want to do it once and our gate.
         if not self.already_cleaned:
             self.already_cleaned = True
             exec_sudo(["kpartx", "-d",
                        self.state['blockdev'][self.base]['device']])
+
+    def cleanup(self):
+        pass