virtio-net: fix wild pointer when remove virtio-net queues

The tx_bh or tx_timer will free in virtio_net_del_queue() function, when
removing virtio-net queues if the guest doesn't support multiqueue. But
it might be still referenced by virtio_net_set_status(), which needs to
be set NULL. And also the tx_waiting needs to be set zero to prevent
virtio_net_set_status() accessing tx_bh or tx_timer.

Cc: qemu-stable@nongnu.org
Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
