virtio: limit avail bytes lookahead

Commit 0d8d7690850eb0cf2b2b60933cf47669a6b6f18f introduced
a regression in virtio-net performance because it looks
into the ring aggressively while we really only care
about a single packet worth of buffers.
Reported as bugzilla 1066055 in launchpad.

To fix, add parameters limiting lookahead, and
use in virtqueue_avail_bytes.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reported-by: Edivaldo de Araujo Pereira <edivaldoapereira@yahoo.com.br>
Tested-by: Edivaldo de Araujo Pereira <edivaldoapereira@yahoo.com.br>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
