nixos modules: cleanup vbox file

nixos-modules/dhall/vbox-guest.dhall

@@ -33,15 +33,13 @@ let toVbox =
           )
       → let vboxConfig =
               ''
-              <?xml version="1.0"?>
               <VirtualBox xmlns="http://www.virtualbox.org/" version="1.14-freebsd">
                 <Machine uuid="{37ab43a5-38d8-4491-93f5-5b0b077f5c32}" name="ubuntu_16_04_64" OSType="Ubuntu_64" snapshotFolder="Snapshots" lastStateChange="2018-01-23T18:40:00Z">
                   <MediaRegistry>
                     <HardDisks>
                       <HardDisk uuid="{${params.vdiUuid}}" location="${params.vdiFilename}" format="VDI" type="Normal"/>
                     </HardDisks>
-                    <DVDImages>
-                    </DVDImages>
+                    <DVDImages/>
                   </MediaRegistry>
                   <Hardware>
                     <CPU count="1">
@@ -58,8 +56,7 @@ let toVbox =
                       <IOAPIC enabled="true"/>
                     </BIOS>
                     <USB>
-                      <Controllers>
-                      </Controllers>
+                      <Controllers/>
                     </USB>
                     <Network>
                       <Adapter slot="0" enabled="true" MACAddress="0800271D7901" cable="true" type="82540EM">
@@ -76,8 +73,7 @@ let toVbox =
                     </LPT>
                     <AudioAdapter controller="HDA" driver="OSS" enabled="false"/>
                     <RTC localOrUTC="UTC"/>
-                    <SharedFolders>
-                    </SharedFolders>
+                    <SharedFolders/>
                   </Hardware>
                   <StorageControllers>
                     <StorageController name="SATA" type="AHCI" PortCount="4" useHostIOCache="true" Bootable="true" IDE0MasterEmulationPort="0" IDE0SlaveEmulationPort="1" IDE1MasterEmulationPort="2" IDE1SlaveEmulationPort="3">