vnc: fix memory leak

If vnc's password is configured, it will leak memory
which cipher variable pointed on every vnc connection.

Cc: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Message-Id: <1437556133-11268-1-git-send-email-arei.gonglei@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
