DVD attachments should never be primary

plugins/kernel_v2/config/disk.rb

@@ -180,6 +180,10 @@ module VagrantPlugins
           errors << I18n.t("vagrant.config.disk.dvd_type_file_required", name: @name, machine: machine.name)
         end
 
+        if @type == :dvd && @primary
+          errors << I18n.t("vagrant.config.disk.dvd_type_primary", name: @name, machine: machine.name)
+        end
+
         if @file
           if !@file.is_a?(String)
             errors << I18n.t("vagrant.config.disk.invalid_file_type", file: @file, machine: machine.name)

templates/locales/en.yml

@@ -1846,6 +1846,9 @@ en:
       disk:
         dvd_type_file_required:
           A 'file' option is required when defining a disk of type `:dvd` for guest '%{machine}'.
+        dvd_type_primary: |-
+          Disks of type ':dvd' cannot be defined as a primary disks. Please
+          remove the 'primary' argument for disk '%{name}' on guest '%{machine}'.
         invalid_ext: |-
           Disk type '%{ext}' is not a valid disk extention for '%{name}'. Please pick one of the following supported disk types: %{exts}
         invalid_type: |-

test/unit/plugins/kernel_v2/config/disk_test.rb

@@ -85,18 +85,24 @@ describe VagrantPlugins::Kernel_V2::VagrantConfigDisk do
     before do
       subject.type = :dvd
       subject.name = "untitled"
-    end
-
-    it "is valid with file path set" do
       allow(File).to receive(:file?).with(iso_path).and_return(true)
       subject.file = iso_path
+    end
+
+    it "is valid with test defaults" do
       subject.finalize!
       assert_valid
     end
 
     it "is invalid if file path is unset" do
+      subject.file = nil
+      subject.finalize!
+      assert_invalid
+    end
+
+    it "is invalid if primary" do
+      subject.primary = true
       subject.finalize!
-      errors = subject.validate(machine)
       assert_invalid
     end
   end