remove VolumeNotFoundForInstance class

VolumeNotFoundForInstance class is used only in test_xiv_ds8k.py.  I
think the exception class that is used only in test code should not be
defined in cinder/exception.py.

Change-Id: I53512d9b06d48e1fb98ef63eac82c033cec37471
Closes-Bug: #1227477
Merge "remove VolumeNotFoundForInstance class"