Examine if GPFS is mounted before writing data.

If GPFS is not mounted, throw an exception and log the error message. That will
prevent create_volume(), copy_image_to_volume() or _clone_image() to create
file on local disk, which can not be removed by the driver code itself.

Change-Id: If5d4c7a88e82362d2a87068202926b1ca82d9dff
Closes-Bug: #1242511
Merge "Examine if GPFS is mounted before writing data."