If there is a live migration of an instance that was launched
before change Iecc265fb25e88fa00a66f1fd38e215cad53e7669, it
would not have an mtu set and therefore it wouldn't have it in
the XML.
When live migrating, the mtu is added which changes the
guest ABI[1], causing the live migration to fail. The failure
occurs when trying to live migrate an instance that:
- Launched before change Iecc265fb25e88fa00a66f1fd38e215cad53e7669
- It has not been rebooted (i.e. XML has not changed since)
- It's using bridge/tap networking
- Migration attempted after change Iecc265fb25e88fa00a66f1fd38e215cad53e7669
This patch prevents this by avoiding setting MTU if the running
instance does not have one configured in its domain XML.
[1]: https://bugzilla.redhat.com/show_bug.cgi?id=1449346
Closes-Bug #1800511
Change-Id: I6e2e6437a7c826dc425d8b353c38670d6eece0b5
643f53f5e9