Add VirtualBox instructions and task to create a VMDK

Rsconscript

@@ -147,6 +147,14 @@ task "run", desc: "Run HOS in QEMU" do
   end
 end
 
+# See README.md for how to set up VirtualBox for HOS.
+task "mk-vmdk", desc: "Create VirtualBox VMDK virtual drive for HOS" do
+  vmdk = hello_env.expand("^/HOS.vmdk")
+  rm vmdk
+  sh %W[VBoxManage internalcommands createrawvmdk -filename #{vmdk} -rawdisk #{File.expand_path(hello_env.expand("^/HOS.img"))}]
+end
+
+# See README.md for how to set up VirtualBox for HOS.
 task "run-vb", desc: "Run HOS in VirtualBox" do
   sh %W[VBoxManage startvm HOS]
 end