drivers: introduce vDPA class

drivers: introduce vDPA class

The vDPA (vhost data path acceleration) drivers provide support for
the vDPA operations introduced by the rte_vhost library.

Any driver which provides the vDPA operations should be moved\added to
the vdpa class under drivers/vdpa/.

Create the general files for vDPA class in drivers and in documentation.

The management tree for vDPA drivers is
git://dpdk.org/next/dpdk-next-virtio.

Signed-off-by: Matan Azrad <matan@mellanox.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>