Check if dir exists before calling listdir

Changes along the way to how we clean up and detach after
copying an image to a volume exposed a problem in the cleanup
of the brick/initiator routines.

The clean up in the initiator detach was doing a blind listdir
of /dev/disk/by-path, however due to detach and cleanup being
called upon completion of the image download to the volume if
there are no other devices mapped in this directory the directory
is removed.

The result was that even though the create and copy of the image
was succesful, the HostDriver code called os.lisdir on a directory
that doesn't exist any longer and raises an unhandled exception that
cause the taskflow mechanism to mark the volume as failed.

Change-Id: I488755c1a49a77f42efbb58a7a4eb6f4f084df07
Closes-bug: #1243980