virtio-net: remove virtio queues if the guest doesn't support multiqueue

commit da51a335 adds all queues in .realize(). But if the
guest doesn't support multiqueue, we forget to remove them. And
we cannot handle the ctrl vq corretly. The guest will hang.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>