vhost: support selective datapath

vhost: support selective datapath

This patch set introduces support for selective datapath in DPDK vhost-user
lib. vDPA stands for vhost Data Path Acceleration. The idea is to support
virtio ring compatible devices to serve virtio driver directly to enable
datapath acceleration.

A set of device ops is defined for device specific operations:

     a. get_queue_num: Called to get supported queue number of the device.

     b. get_features: Called to get supported features of the device.

     c. get_protocol_features: Called to get supported protocol features of
        the device.

     d. dev_conf: Called to configure the actual device when the virtio
        device becomes ready.

     e. dev_close: Called to close the actual device when the virtio device
        is stopped.

     f. set_vring_state: Called to change the state of the vring in the
        actual device when vring state changes.

     g. set_features: Called to set the negotiated features to device.

     h. migration_done: Called to allow the device to response to RARP
        sending.

     i. get_vfio_group_fd: Called to get the VFIO group fd of the device.

     j. get_vfio_device_fd: Called to get the VFIO device fd of the device.

     k. get_notify_area: Called to get the notify area info of the queue.

Signed-off-by: Zhihong Wang <zhihong.wang@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>