finish migration should handle exception to revert instance info

[jichenjc]

when instance resize, it will call finish_migration at last to create new instance and destroy old instance
if driver layer has problem in create new instance, the instance will be set to 'ERROR' state

we are able to use reset-state --active to reset the instance and use it
but the instance information is set to new instance and not reverted to old one

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/83278

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/83278
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=e686131fc4b8724328f0922067569120c90eb261
Submitter: Jenkins
Branch: master

commit e686131fc4b8724328f0922067569120c90eb261
Author: jichenjc <email address hidden>
Date: Tue Mar 25 06:17:01 2014 +0800

    Restore instance flavor info when driver finish_migration fails

    when instance resize, it will call finish_migration at last to create
    new instance and destroy old instance. If driver layer has problem in
    create new instance, the instance will be set to 'ERROR' state.

    we are able to use reset-state --active to reset the instance and use it
    but the instance information is set to new flavor
    and not reverted to old one

    Change-Id: I5961260f50e7893c7bf03e329e06edfd4f295640
    Closes-Bug: #1296519