vhost-net: save & restore vhost-user acked features

vhost-net: save & restore vhost-user acked features

The initial vhost-user connection sets the features to be negotiated
with the driver. Renegotiation isn't possible without device reset.

To handle reconnection of vhost-user backend, ensure the same set of
features are provided, and reuse already acked features.

Signed-off-by: Marc-André Lureau <[email protected]>
Tested-by: Yuanhan Liu <[email protected]>
Reviewed-by: Yuanhan Liu <[email protected]>
Reviewed-by: Victor Kaplansky <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>