Previously errors while disconnecting volumes from the source host
during post_live_migration within LibvirtDriver would result in the
overall failure of the migration. This would also mean that while the
instance would be running on the destination it would still be listed as
running on the source within the db.
This change simply ignores any exceptions raised while attempting to
disconnect volumes on the source. These errors can be safely ignored as
they will have no impact on the running instance on the destination.
In the future Nova could wire up the force and ignore_errors kwargs when
calling down into the associated os-brick connectors to help avoid this.
Closes-Bug: #1843639
Change-Id: Ieff5243854321ec40f642845e87a0faecaca8721
ac68cffd43