Description of problem:
The current default behavior during a cinder-backup/cinder-restore procedure is that thin volumes are restored as thick. Taking much more space than necessary and creating problems during migration.

It would be good if the original volume size is kept during backup/restore procedure.

Version-Release number of selected component (if applicable):
rhosp13/openstack-cinder-api:124

How reproducible:
100%

Steps to Reproduce:
1.Create empty volume (10GB size but 0B used)
2.Cinder-backup to export the volume
3.Cinder-restore to import the volume
4.Volume now uses 10GB of disk space

Actual results:
Thin volume become thick

Expected results:
Thin volume stay thin

Additional info: