Fix migration and instance resize update order

This commit switches the order in which migrations and instances are
updated when resizing an instance. Previously, if you set
resize_confirm_window to some small value, your resize instance could go
into error state because the periodic task was run after the migration
was updated but before the instance object was updated. This change makes it
so the instance is always updated before the migration.

The _test_finish_resize test case has also been refactored to enforce
saving the instance object before the migration object. In the event
that the instance object is saved after the migration, like the previous
implementation, the test cases will fail since the states of the
migrations will be out of sync in the _mig_save assertions.

Closes-Bug: #1321298
Change-Id: I0490fc4b4e03b36eb01a06a95ea761f4cf8df469
Merge "Fix migration and instance resize update order"