Add restore_volume_id in backup

Add restore_volume_id in backup

This patch is to add restore_volume_id in backup object.
When restoring a volume from a backup, it saves the
volume in backup object.

Currently volume service and backup service are in same host.
When backup service starts, it does cleanup tasks on both
backups and volumes on current host.

But with bp scalable-backup-service, backup service and
volume services can run on different hosts. When doing cleanup
tasks, we need to find out backing-up and restoring volumes
related to the backups on current host. Backing-up volumes can
be found with field backup.volume_id. Restoring volumes are found
by new field backup.restore_volume_id.

Change-Id: I757be7a5e47fc366c181400587b5a61fe3709a0b
Partial-Implements: bp scalable-backup-service
Co-Authored-By: Tom Barron <tpb@dyncloud.net>