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
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"