As qemu_new_net_client create new ncs but error happens later,
ncs will be left in global net_clients list and we can't use them any
more, so we need to cleanup them.
Cc: qemu-stable@nongnu.org
Signed-off-by: linzhecheng <linzhecheng@huawei.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>