migration: Fix cancel state

migration: Fix cancel state

During a cancelled migration there's a race where the fd can
go into an error state before we get back around the migration loop
and migration_detect_error transitions from cancelling->failed.

Check for cancelled/cancelling and don't change the state.

Red Hat bug: https://bugzilla.redhat.com/show_bug.cgi?id=1608649

Fixes: b23c2ade250
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190219195928.12289-1-dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>