Keep Migration status in automatic confirm-resize

function in compute/manager.py _poll_unconfirmed_resizes
will translate the migrate status from finished to error
whenever finds a problem ,Consider the following case

1) _poll_unconfirmed_resizes runs and founds several migrations to be confirmed
2) user deletes the instance, so its task state will be changed to DELETING
3) _poll_unconfirmed_resizes will find the task state is not None,
it will make the migration error status
4) following code in _delete
if instance.vm_state == vm_states.RESIZED:
self._confirm_resize_on_deleting(context, instance)
will fail because migration status already updated

User is able to do the migration operations when they found error
so we don't need to handle this kind of error for them

Change-Id: Ia79dd56c9c1fa8add7dc138d838cbfbac4523ac3
Closes-Bug: #1301696
Merge "Keep Migration status in automatic confirm-resize"