vhost_net: init acked_features to backend_features

vhost_net: init acked_features to backend_features

commit 2e6d46d77ed328d34a94688da8371bcbe243479b (vhost: add
vhost_get_features and vhost_ack_features) removes the step that
initializes the acked_features to backend_features.

As this field is now uninitialized, vhost initialization will sometimes
fail.

To fix, initialize acked_features on each ack.

Tested-by: Andrey Korolyov <[email protected]>
Cc: Nikolay Nikolaev <[email protected]>
Cc: [email protected]
Signed-off-by: Jason Wang <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>