Extend storages to support more storages and of different types

[Freddo3000]

This PR aims to make it possible for VMs to mount several disks and of different types. So far I've implemented File and RBD, but it should be fairly easy to extend to other libvirt storages. Opening it as a draft here for feedback as I continue working on it.

Example job definition:

job "virt-example" {

  group "virt-group" {
    count = 1

    task "virt-task" {

      identity {
        env  = true
        file = true
      }

      template {
        data        = <<EOH
        Guest System
        EOH
        destination = "local/index.html"
      }

      driver = "nomad-driver-virt"

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

      config {
        default_user_password           = "password"
        cmds                            = ["touch /home/ubuntu/file.txt"]
        default_user_authorized_ssh_key = "ssh-ed25519 AAAAC3NzaC1lZDI..."

        disks {
          file "vda" {
            path = "local/focal-server-cloudimg-amd64.img"
          }
          rbd "vdb" {
            name = "pool/my-rbd"
            auth {
              username = "libvirt"
              uuid = "<uuid>"
            }
            host "ceph01.example.org" {
              port = 6789
            }
            host "ceph02.example.org" {
              port = 6789
            }
          }
        }
      }

      resources {
        cores  = 4
        memory = 4000
      }
    }
  }
}

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