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 "Grab instance for migration before updating usage"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp