migration: fix id leak regression

This leak was introduced in commit
581f08bac22bdd5e081ae07f68071a0fc3c5c2c7.

(it stands out quickly with ASAN once the rest of the leaks are also
removed from make check with this series)

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170221141451.28305-31-marcandre.lureau@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
