Add ability to update migration info on backend

The current migration process creates a new volume,
xfr's it's contents, then deletes the original and
modifies the new volume to have the previous ID.

All in all this is kinda troublesome, but regardless
the bigger problem is that the generic impl doesn't
provide any method to tell the backend devices that
their device names/id's have changed.

This patch provides a method to inform backends
that a migration operation has been completed on
their target volume.

It shouldn't be necessary to do anything with the originating
or source volume because it's deleted as part of the process.

Change-Id: Ib5e6a47fe9eedab3e60e77a6c2d987355c0bf167
Closes-Bug: #1383499