Keep migration status if instance still resizing

There is a race condition during finish_resize() where
poll_unconfirmed_resizes() can get called between the database save
of the migration record and instance record. The migration record
will be marked 'finished', enabling poll_unconfirmed_resizes() to pick
it up. But the instance will not have been saved yet to have the
vm_state of RESIZED, and task_state of None. The task_state will
still be RESIZE_FINISH. This will cause poll_unconfirmed_resizes() to
put the migration record in error state.

Instead, poll_unconfirmed_resizes() should ignore instances with
a task state of RESIZE_FINISH and not put the migration record
into error state.

Change-Id: I825234de5e4a4f0cc906ac05b715ed85cd953aa1
Closes-Bug: #1376933
(cherry picked from commit 0a84bb80c2ea66a806395964684e07d7bc6accbd)
Merge "Keep migration status if instance still resizing" into stable/juno