/v/vm/efi can't attach "BLOCK" media

[zimsneexh]

Sending a /v/vm/efi PUT request with the following media field:

...
"media": [
    {
      "mid": 0,
      "mode": "BLOCK",
      "readonly": false
    }
  ],
...

causes an error on the server side and doesn't create the VM.

[maxkofler]

The root of the problem is quite simple: Velocity currently doesn't support BLOCK devices:

enum DiskMode : String, Decodable, Encodable {
    case USB = "USB"
    case VIRTIO = "VIRTIO"
}

We should rename this issue to implement that functionality.