vhost: check for ring descriptors overflow
vhost: check for ring descriptors overflow

A malicious guest may easily forge some illegal vring desc buf.
To make our vhost robust, we need make sure desc->next will not
go beyond the vq->desc[] array.

Suggested-by: Rich Lane <01fa1ed5f6b7994778e75c34db4e5eae84b07b5a@bigswitch.com>
Signed-off-by: Yuanhan Liu <5dcef21195e64c9c08aa779bb62b25e9133eaa33@linux.intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk