Need to scSaveInstanceInfo() after volume detach

[iain MacDonnell]

When a volume is attached to an instance, scSaveInstanceInfo() is called to update the stored instance data, but when a volume is detached, this saved data is not updated. If a volume is attached then detached, then the NC is restarted, subsequent attempts to attach the same volume will fail because the NC thinks it's already there. Symptom error:

[Wed Dec 1 08:38:47 2010][030863][EUCAFATAL ] ERROR: Failed to save the volume record, aborting volume attachment
[Wed Dec 1 08:38:47 2010][030863][EUCAERROR ] ERROR: doAttachVolume() failed error=1

I believe there needs to be a scSaveInstanceInfo() call after free_volume() in doDetachVolume()