GlusterFS backup support

[Eric Harney]

Changes at https://review.openstack.org/#/c/70201/ and https://review.openstack.org/#/c/70405/ enable backups for GlusterFS.

However, in current form, this will only work correctly for volumes that don't have snapshots.

For the GlusterFS driver, we should
a) Initially fail backups for volumes with snapshots so the above can proceed
b) Add support for backing up volumes with snapshots by converting the qcow2 data into a usable form at backup time.

See also: bug 1247743 for NFS backup support.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/72036

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/72036
Committed: https://git.openstack.org/cgit/openstack/cinder/commit/?id=41e3a94ae18a9219ff4ab10389f3a7799c9b6493
Submitter: Jenkins
Branch: master

commit 41e3a94ae18a9219ff4ab10389f3a7799c9b6493
Author: Eric Harney <email address hidden>
Date: Fri Feb 7 12:58:35 2014 -0500

    GlusterFS: Fix create/restore backup

    Allow create_backup for GlusterFS volumes, but
    only when snapshots do not exist. (For now.)

    Restore is a no-op for the driver, so allow it.

    Related-Bug: 1247743
    Closes-Bug: 1275977

    Change-Id: I50b8f6cac684c967c7374bb43247a396ce936157