GlusterFS: Set permissions on qcow2 snapshot files

GlusterFS: Set permissions on qcow2 snapshot files

The GlusterFS driver sets specific permissions on the
volume file when volumes are created.  Setting them on the
snapshot files at snapshot creation time is also required,
otherwise volume attaches will fail for volumes with snapshots
when using QEMU's libgfapi functionality.

This does not weaken the current security model in any
significant way, as it only lines up the permissions used
for snapshot files with those already used for volume files.

This will eventually be addressed by functionality in libvirt:
https://bugzilla.redhat.com/show_bug.cgi?id=1052014

Closes-Bug: #1286376

Change-Id: I93944724c557c907eb3821a061e7434f09ff20bc