veyron/services/mgmt/device/impl: refactor Update method for updating instances
veyron/services/mgmt/device/impl: refactor Update method for updating instances

This minor code refactoring is laying the groundwork for implementing
per-instance updates.

Change-Id: I0a1c0af3c66d95f4ded025e5dfc633b5090c3982

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref