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 <andrey@xdel.ru>
Cc: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
vhost-user: fix VIRTIO_NET_F_MRG_RXBUF negotiation

Header length check should happen only if backend is kernel. For user
backend there is no reason to reset this bit.

vhost-user code does not define .has_vnet_hdr_len so
VIRTIO_NET_F_MRG_RXBUF cannot be negotiated even if both sides
support it.

Signed-off-by: Damjan Marion <damarion@cisco.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>