vhost-vdpa: support get/set backend features

This patch makes userspace can get and set backend features to
vhost-vdpa.

Signed-off-by: Cindy Lu <lulu@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20200804162048.22587-4-eli@mellanox.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>