migration: Our release callback was just free

migration: Our release callback was just free

We called it from a single place, and always with state !=
MIG_STATE_ACTIVE.  Just remove the whole callback.  For users of the
notifier, notice that this is exactly the case where they don't care,
we are just freeing the state from previous failed migration (it can't
be a sucessful one, otherwise we would not be running on that machine
in the first place).

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>