Explain why the give methods are whitelisted.

In order to aid mine (and likely others) understanding of this
code it seems to make sense to document what these special whitelisted
attributes are.

Change-Id: I1c11484fc74e898df5d25ab94ed0cdb288accd67
Grab instance for migration before updating usage

Updates _update_usage_from_migrations so that it grabs the instance
associated with a migration before updating usage.

Bug 1163622

Change-Id: I64acef912ae71ea5c7fb66e9fcaafe5750c63eaf
Merge "Explain why the give methods are whitelisted."
Fix a typo in attach_interface error path

This prevents an attach failure from raising the proper exception to
the caller.

Change-Id: I51559cf395fc99738127b7a443437197d15f3d4c
Merge "Grab instance for migration before updating usage"
Merge "Fix a typo in attach_interface error path"