vhost: generialize backend features setting/getting

Move the backend features setting/getting from net.c to vhost.c to be
reused by vhost-vdpa.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20200804162048.22587-3-eli@mellanox.com


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