runstate/migrate: Two more transitions

There's a race if someone does a 'stop' near the end of migrate;
the migration process goes through two runstates:
    'finish migrate'
    'postmigrate'

If the user issues a 'stop' between the two we end up with invalid
state transitions.
Add the transitions as valid.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20170804175011.21944-1-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>
