examples/vhost_scsi: introduce a new sample app

examples/vhost_scsi: introduce a new sample app

vhost-user protocol is common to many virtio devices, such as
virtio_net/virtio_scsi/virtio_blk. Since DPDK vhost library
removed the NET specific data structures, the vhost library
is common to other virtio devices, such as virtio-scsi.

Here we introduce a simple memory based block device that
can be presented to Guest VM through vhost-user-scsi-pci
controller. Similar with vhost-net, the sample application
will process the I/Os sent via virt rings.

Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>