Disable virtio offloading feature bits

This fixes the compatibility with vmnet framework defaults, and shows
better and more consistent performance in most cases.

vmnet has partly documented vmnet_enable_tso_key and
vmnet_enable_checksum_ofload_key options. Enabling these options allows
vment helper to work with current krunkit.

Before (TSO and checksum offload enabled):

| network              | vm       | iper3         | iperf3 -R     |
|----------------------|--------- |---------------|---------------|
| gvproxy              | krunkit  |  1.40 Gbits/s | 20.00 Gbits/s |
| vmnet-helper shared  | krunkit  |  1.38 Gbits/s | 46.20 Gbits/s |
| vmnet-helper bridged | krunkit  |  1.37 Gbits/s | 45.70 Gbits/s |
| vmnet-helper shared  | vfkit    |  4.27 Gbits/s |  8.09 Gbits/s |
| vmnet-helper bridged | vfkit    |  4.30 Gbits/s | 10.50 Gbits/s |

After (TSO and checksum offload disabled):

| network              | vm       | iper3         | iperf3 -R     |
|----------------------|----------|---------------|---------------|
| gvproxy              | krunkit  |  1.47 Gbits/s |  2.58 Gbits/s |
| vmnet-helper shared  | krunkit  | 10.10 Gbits/s |  8.38 Gbits/s |
| vmnet-helper shared  | krunkit  | 10.10 Gbits/s |  8.38 Gbits/s |
| gvproxy              | vfkit    |  1.43 Gbits/s |  2.84 Gbits/s |
| vmnet-helper shared  | vfkit    | 10.70 Gbits/s |  8.41 Gbits/s |
| vmnet-helper bridged | vfkit    | 12.10 Gbits/s | 11.50 Gbits/s |

It seems that using offloading makes sense only in special cases, so it
should be optional.

Part-of: containers#264

src/devices/src/virtio/net/device.rs

@@ -21,10 +21,7 @@ use std::path::PathBuf;
 use std::sync::atomic::AtomicUsize;
 use std::sync::Arc;
 use utils::eventfd::{EventFd, EFD_NONBLOCK};
-use virtio_bindings::virtio_net::{
-    VIRTIO_NET_F_CSUM, VIRTIO_NET_F_GUEST_CSUM, VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_UFO,
-    VIRTIO_NET_F_HOST_TSO4, VIRTIO_NET_F_HOST_UFO, VIRTIO_NET_F_MAC,
-};
+use virtio_bindings::virtio_net::VIRTIO_NET_F_MAC;
 use virtio_bindings::virtio_ring::VIRTIO_RING_F_EVENT_IDX;
 use vm_memory::{ByteValued, GuestMemoryError, GuestMemoryMmap};
 
@@ -93,15 +90,8 @@ pub struct Net {
 impl Net {
     /// Create a new virtio network device using the backend
     pub fn new(id: String, cfg_backend: VirtioNetBackend, mac: [u8; 6]) -> Result<Self> {
-        let avail_features = 1 << VIRTIO_NET_F_GUEST_CSUM
-            | 1 << VIRTIO_NET_F_CSUM
-            | 1 << VIRTIO_NET_F_GUEST_TSO4
-            | 1 << VIRTIO_NET_F_HOST_TSO4
-            | 1 << VIRTIO_NET_F_GUEST_UFO
-            | 1 << VIRTIO_NET_F_HOST_UFO
-            | 1 << VIRTIO_NET_F_MAC
-            | 1 << VIRTIO_RING_F_EVENT_IDX
-            | 1 << VIRTIO_F_VERSION_1;
+        let avail_features =
+            1 << VIRTIO_NET_F_MAC | 1 << VIRTIO_RING_F_EVENT_IDX | 1 << VIRTIO_F_VERSION_1;
 
         let mut queue_evts = Vec::new();
         for _ in QUEUE_SIZES.iter() {