Description of problem:
GlusterFS volume/snapshot create/delete/extend operations need to use locking to prevent multiple threads from modifying on-disk data at the same time.

Version-Release number of selected component (if applicable):
2013.2

How reproducible:
Easily with the tweaks below, not often in general.

Steps to Reproduce:
To demonstrate this, add a time.sleep(15) call inside of _create_snapshot() immediately after the call self._create_qcow2_snap_file() and run many snapshot-create operations at once, then attempt to delete all of the snapshots that have been created.

Actual results:
Some of the snapshot delete operations fail.

Expected results:
All snapshot delete operations succeed.