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
Merge "Restore instance flavor info when driver finish_migration fails"