hw/net/xen_nic.c: Free 'netdev->txs' when map 'netdev->rxs' fails

When map 'netdev->rxs' fails, need free the original resource, or will
cause resource leak.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
