GlusterFS: Synchronize additional operations

create_cloned_volume() and copy_volume_to_image() should also be
locked against other driver operations to prevent collisions
from concurrent updates of snapshot metadata or volume file data.

Closes-Bug: #1267983

Change-Id: Idd6158bfbe4dc049c33fa58b40d3d570fc244d01
Merge "GlusterFS: Synchronize additional operations"