Revert "virtio_ring: validate used buffer length"

Revert "virtio_ring: validate used buffer length"

This reverts commit f575a3e0ac9510b4ff84382a04c6bf15ccd12501.

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>