Revert "virtio_ring: validate used buffer length"

This reverts commit 939779f5

.

Attempts to validate length in the core did not work out: there turn out
to exist multiple broken devices, and in particular legacy devices are
known to be broken in this respect.

We have ideas for handling this better in the next version but for now
let's revert to a known good state to make sure drivers work for people.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>