net/virtio: introduce vhost-vDPA backend

net/virtio: introduce vhost-vDPA backend

vhost-vDPA is a new virtio backend type introduced by vDPA kernel
framework, which provides abstraction to the vDPA devices and
exposes an unified control interface through a char dev.

This patch adds support to the vhost-vDPA backend. As similar to
the existing vhost kernel backend, a set of virtio_user ops were
introduced for vhost-vDPA backend to handle device specific operations
such as:
 - device setup
 - ioctl message handling
 - queue pair enabling
 - dma map/unmap
vDPA relevant ioctl codes and data structures are also defined in
this patch.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>