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
The following error was output:
```
vyos@vyos# load /usr/libexec/vyos/tests/config/bgp-evpn-l3vpn-pe-router
Loading configuration from '/usr/libexec/vyos/tests/config/bgp-evpn-l3vpn-pe-router'
Migration script error: /opt/vyatta/etc/config-migrate/migrate/interfaces/20-to-21: [Errno 1] failed to run command: ['/opt/vyatta/etc/con
fig-migrate/migrate/interfaces/20-to-21', '/tmp/tmprr8xf78f']
- op: set path: ['interfaces', 'ethernet', 'eth0', 'offload', 'gro'] value: None replace: True
- op: set path: ['interfaces', 'ethernet', 'eth0', 'offload', 'gso'] value: None replace: True
- op: set path: ['interfaces', 'ethernet', 'eth0', 'offload', 'sg'] value: None replace: True
- op: set path: ['interfaces', 'ethernet', 'eth0', 'offload', 'tso'] value: None replace: True
- op: set path: ['interfaces', 'ethernet', 'eth1', 'offload', 'gro'] value: None replace: True
- op: set path: ['interfaces', 'ethernet', 'eth1', 'offload', 'gso'] value: None replace: True
- op: set path: ['interfaces', 'ethernet', 'eth1', 'offload', 'sg'] value: None replace: True
- op: set path: ['interfaces', 'ethernet', 'eth1', 'offload', 'tso'] value: None replace: True
- op: set path: ['interfaces', 'ethernet', 'eth2', 'offload', 'gro'] value: None replace: True
- op: set path: ['interfaces', 'ethernet', 'eth2', 'offload', 'gso'] value: None replace: True
- op: set path: ['interfaces', 'ethernet', 'eth2', 'offload', 'sg'] value: None replace: True
- op: set path: ['interfaces', 'ethernet', 'eth2', 'offload', 'tso'] value: None replace: True
exit code: 1.
```