GlusterFS: Synchronize operations that manipulate qcow2 data

Operations that modify qcow2 chains or the GlusterFS driver's
snapshot info file should use locking so that more than one thread
cannot run these operations at the same time.

Without this it is possible for multiple threads to concurrently
modify snapshot information resulting in an incomplete snapshot
chain.

Closes-Bug: #1251425

Change-Id: I93c6ffecd9aec38560cb19085243390b9120b363
Merge "GlusterFS: Synchronize operations that manipulate qcow2 data"