hw/virtio/virtio: Don't allow guests to add/remove queues

A queue size of 0 is used to indicate a nonexistent queue, so
don't allow the guest to flip a queue between zero-size and
non-zero-size. Don't permit setting of negative queue sizes
either.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1374853288-9912-2-git-send-email-peter.maydell@linaro.org
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
