crash on hot-unplug of vmxnet3

Hot-unplug of a vmxnet3 device crashes as follows:

(qemu) device_add id=ff,driver=vmxnet3
[vmxnet3][WR][vmxnet3_peer_has_vnet_hdr]: Peer has no virtio extension. Task offloads will be emulated.
(qemu) device_del ff
(qemu) qemu-system-x86_64: /home/pbonzini/work/upstream/qemu/net/net.c:315: qemu_del_net_client: Assertion `queues != 0' failed.