Change arguments to volume_detach()

Most compute API calls accept an instance object.  To be compatible with
cells, volume_detach() should as well.  Also change it to accept the
volume object, instead of just a volume ID.  This lets us proxy the
volume object to child cells a bit more easily.

This removes 2 skipTests that we have related to volumes and cells.

Fixes bug 1157408

There was also a check that if instance_get_by_uuid() returned no
instance, it VolumeUnattached should be raised.  However, this check
would never work as InstanceNotFound would have been raised.  This
patch also corrects that.

Change-Id: Ide35ad5d8e147f6bd21b5840b901363dab0eae38
Merge "Change arguments to volume_detach()"