interfaces: T6519: harden config migration if ethernet interface is missing

During a corner case where the configuration is migrated to a different system
with fewer ethernet interfaces, migration will fail during an image upgrade.

vyos.ethtool.Ethtool() is instantiated with an invalid interface leading to an
exception that kills the migrator

(cherry picked from commit e47d4fd385631236da6882233b09f6364cbb077b)