Fix retype failure when original has no volume type

Fix retype failure when original has no volume type

cinder.objects.volume.Volume.finish_volume_migration()
should skip the volume_type_id field when swapping fields
between "source" and "dest" volumes.  The "dest" volume
has already been created with appropriate volume_type_id
and the "source" may have not have a volume type.

This commit also changes a LOG.error() to a LOG.exception()
in where finish_volume_migration is called in order to
show the exception traceback in its calling context.

Change-Id: I2caf4d3f4aa088d099548e6e88d1776b4cc5810c
Closes-bug: #1547546