Implement macvtap interface configuration

[Freddo3000]

This pull request implements the macvtap interface configuration. As a macvtap interface typically has it's own IP, it is not really compatible with nomad ports definitions, as such they're omitted for this mode. CNI with a dummy interface could be used instead.

I'm leaving this as a draft to gather feedback until I've tested it a bit.

[Freddo3000]

I've now tested it with bridge networking and hosts are able to acquire IPs via DHCP. It can also acquire a static IP by passing it through cloud-init.

job "server" {
  group "virt-group" {
    reschedule {
      attempts  = 0
      unlimited = false
    }

    task "virt-task" {
      driver = "virt"

      artifact {
        source      = "http://cloud-images.ubuntu.com/noble/current/noble-server-cloudimg-amd64.img"
        destination = "local/noble-server-cloudimg-amd64.img"
        mode        = "file"
      }

      config {
        user_data =<<EOH
#cloud-config
password: secret
ssh_pwauth: true
EOH
        network_interface {
          macvtap {
            device  = "ens1.320"
          }
        }
        
        disk {
          size    = "10GiB"
          pool    = "ceph"
          chained = true
          source {
            image = "local/noble-server-cloudimg-amd64.img"
          }
        }
      }
      
      resources {
        cores = 4
        memory = 4096
      }
    }
  }
}

I have not tested any of the other macvtap modes other than bridge as I don't have any equipment available to test it on properly, but I don't see why it wouldn't work.

[chrisroberts]

Hi @Freddo3000, thanks for this PR! Pulled in your changes and everything works as described. This looks great. Left a couple comments, and it would be great to get a couple tests added in here to verify the config. We're lacking tests elsewhere so I'm happy to add coverage related to this as we create those tests.

[chrisroberts]

So there is a helper for these checks and after importing the github.com/hashicorp/nomad-driver-virt/internal/errs package, the empty string conditional can be removed and this can be updated to:

mErr = multierror.Append(mErr, errs.MissingAttribute("network_interface.macvtap.device", netInterface.Macvtap.Device))

However, I see that the validation here isn't using a pointer for the mErr value so using the multierror.Append helper isn't possible! I'll update this validation method in a subsequent PR 🙂

[chrisroberts]

The default mode can be included in the spec:

...
"mode": hclspec.NewDefault(
  hclspec.NewAttr("mode", "string", false),
  hclspec.NewLiteral(fmt.Sprintf("%q", MacvtapModeBridge)),
),
...

[chrisroberts]

How about updating this name to Configurable so that it's not limited to only bridge devices?