virtio-net: unbreak the minix guest

Multiqueue patchset conditionally add control vq only when guest negotiate the
feature. Though the spec is not clear on this but it breaks the minix guest
since it will identify the ctrl vq even if it does not support it. Though this
behavior seems a violation on the spec "If the VIRTIO_NET_F_CTRL_VQ feature bit
is negotiated, identify the control virtqueue.", to keep the backward
compatibility, always add the ctrl vq at end of the queues.

Reported-by: Aurelien Jarno <aurelien@aurel32.net>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-id: 1366874663-2566-1-git-send-email-jasowang@redhat.com
Cc: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
