Add backup volume size to database

Currently the process to restore a backup involves creating a new instance, which in turn requires an optional volume size.

If the disk utilization of the backup is actually larger than the volume requested then the create call should raise a 400 error.

During a backup we need to record the disk utilization and store that in the backups table. Then use this field during the create instance call if a backup id is specified.

The api for listing backups and backup details should return this size as well.

This will require a change to the documentation for the list api and a new error code/exception for the create instance call.