mitter Michael S. Tsirkin <mst@redhat.com> 1311234507 +0300

vhost: handle wrap around in # of bufs math

The meth for calculating the # of outstanding buffers gives
incorrect results when vq->upend_idx wraps around zero.
Fix that.

Signed-off-by: Shirley Ma <xma@us.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
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