virtio: add the checks for descriptors chain len

virtio: add the checks for descriptors chain len

Type: fix

virtio uses indirect descriptors for chain buffers.
indirect descriptor chain is mapped on a vlib_buffer_t.
Single descriptor is 16 bytes and vlib_buffer_t has
2048 bytes space. So maximum long chain can have
128 (=2048/16) indirect descriptors.

This patch adds check to make sure descriptors chain
len should not exceed 128.

Signed-off-by: Mohsin Kazmi <sykazmi@cisco.com>
Change-Id: I060cfb7709568f42c9b5634527172690ce66a1a3