virtio: error out if guest exceeds virtqueue size

virtio: error out if guest exceeds virtqueue size

A broken or malicious guest can submit more requests than the virtqueue
size permits.

The guest can submit requests without bothering to wait for completion
and is therefore not bound by virtqueue size.  This requires reusing
vring descriptors in more than one request, which is incorrect but
possible.  Processing a request allocates a VirtQueueElement and
therefore causes unbounded memory allocation controlled by the guest.

Exit with an error if the guest provides more requests than the
virtqueue size permits.  This bounds memory allocation and makes the
buggy guest visible to the user.

Reported-by: Zhenhao Hong <zhenhaohong@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>