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

(cherry picked from commit 8a1fd8889170c289910944db016a4206fe5e1e32)
Change-Id: I93c6ffecd9aec38560cb19085243390b9120b363
GlusterFS: Set correct permissions for volume file created via clone

This is currently done when creating a new volume from scratch,
but not when cloning a new volume.

(cherry picked from commit 9c816aa0cf20ff0200831f58d8f14efb5e9e288e)
Closes-Bug: #1254768
Change-Id: I8506dc0824bacdb6cc42b4cadd41c2cad3a74b5b
Merge "GlusterFS: Synchronize operations that manipulate qcow2 data" into stable/havana
Merge "GlusterFS: Set correct permissions for volume file created via clone" into stable/havana