Always store old instance_type during a migration

Previously, we were trying to optimize for the migration (as opposed
to resize) case by not storing the old_ instance_type. However,
this is confusing to the vast amount of shared code that works on
migrations and resizes because the "is old == new" logic must be
replicated everywhere. To cut our losses and hopefully avoid other
hidden dragons, this patch removes the optimization and makes both
paths the same.

Fixes bug #1160489

Change-Id: I2ad7b02239baada5fd2174f5b61cdc90ec91e6f5
(cherry picked from commit 985d703d2731fe9ec968ef6698bbf39743ede32a)