xenapi: Remove unecessary exception handling

The volume code was catching exceptions from the XenAPI only to re-raise
more-generic StorageError exceptions. This practice makes it much more
difficult to discover the root-cause of issues in production by making
the message in the instance-faults table less useful.

The solution is to have the original root-cause exception propogate all
the way up to the instance-faults table where the developer will be able
to make sense of it.

Other cleanups:

* Remove useless logging and stop logging opaque-refs in some places

* Remove dead code (create_iscsi_storage)

* Remove one-off functions (forget_sr_if_present,
  introduce_sr_unless_present); just use forget_sr and introduce_sr

* Removed unused `exception` import

* Added Openstack copyright lines

Future Work:

There are lots of other places where unecessary exception handling is
occuring; this patch just addresses this in the attach/detach code-path.

Fixes bug 1122733

Change-Id: I8c382fb505303e604ff2e86afcf302efe3d6851d

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