migrate: Fix cancelling state warning

We've been getting the warning:

migration_iteration_finish: Unknown ending state 2

on a cancel.

I think that's originally due to 39b9e179;  although
I've only seen the warning, I think that in some cases
that we could find the VM stays paused after a cancel where
it should restart.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20180719092257.12703-1-dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>