Revert "virtio_ring: validate used buffer length"

Revert "virtio_ring: validate used buffer length"

This reverts commit e4506783420ae833cff145e5cd7eb5489d9d020b.

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>