virtio-spec: fix two typos

VIRTIO_NET_F_VTRL_VQ -> VIRTIO_NET_F_CTRL_VQ
VIRTIO_NET_CTRL_MQ is defined to 4 in kernel code

Signed-off-by: Amos Kong <akong@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>