vhost-net: create vhost based Net backend

[majek]

Changes

vhost-net: create vhost based Net backend

This patch adds a second backend to Net devices. It can be enabled with 'vhost'
bool on the network interface config, like this:

"network-interfaces": [
    {
        "iface_id": "eth0",
        "host_dev_name": "tap0",
        "vhost": true
    }
],

Vhost backend opens host kernel /dev/vhost-net interface, and performs
a setup dance to setup the vhost device with the relevant tap
interface. The effect is that all of the data plane goes directly
between host kernel and the guest. The data doesn't go via
firecracker VMM at all. This drastically reduces the packet latency
and increases throughput, especially in a high-pps scenarios. For
example UDP and TCP without offloads.

The control plane is somewhat hacky. Technically, the interrupts from
host to guest should go through firecracker VMM, but this is avoidable
by splicing the host eventfd into the guest interruptfd, and
force-returning VIRTIO_MMIO_INT_VRING in the relevant virtio register.

There are couple of missing features:

• persist (no blockers, just work)
• mmds (no obvious way to do it, perhaps possible with ebpf)
• rate_limiting (no obvious way to implement it, perhaps with ebpf)
• tap/vhost feature negotiation

On the latter point, it would be nice to negotiate some more advanced
tap/vhost features, like USO (UDP segmentation offload), TCP offloads
(flag needed if guest wants to use XDP), VIRTIO_NET_F_MRG_RXBUF (this
might be useful for performance, but benchmarks needed first). Right
now there is no way to express these toggles in the net config, but
this can be done in the future.

Reason

Discussion #3707

License Acceptance

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PR Checklist

• If a specific issue led to this PR, this PR closes the issue.
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this
  PR.
• API changes follow the Runbook for Firecracker API changes.
• User-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.
• New TODOs link to an issue.
• Commits meet
  contribution quality standards.

---

• This functionality cannot be added in rust-vmm.

[majek]

Up for discussion: testing, the errors are wrapped simpler than block-vhost-user, benchmarks, regenerate bindgen.sh to avoid declaring stuff like VIRTIO_NET_F_GUEST_USO4