When managing snapshots, the driver is not checking if the export
exists on backend.
This patch fixes this issue by checking the export on backend. A new
export is created if required.
Change-Id: I2da32016ef99bb307a9e075de0472d56a4817e3c
Depends-On: Ie0ce3c4f2e7e29bf82f5c09a80a3fb132b0481a8
Closes-bug: #1660415
