Raises NotImplementedError for LVM migration.

Currently migration of an LVM backed instance results in a new
instance being launched on the destination node, but the instance
disk is neither copied to the destination nor deleted on the
origin node. The problem is addressed by raising a NotImplementedError.

Closes-Bug: #1270305
Closes-Bug: #1245595
Closes-Bug: #1241866
Change-Id: I8010230b1aa5ddc322d0c93dd916b7700c25ab81
DocImpact
Merge "Raises NotImplementedError for LVM migration."