Now to disalbe/hide Menubar and Stautsbar open terminal and type:
VBoxManage setextradata global GUI/Customizations noMenuBar,noStatusBar
where guest_os_name is the name of the virtual machine. To see the name or UID you cau use:
VBoxManage list vms
After running the first command your VirtualBox guest window will have no Menubar or Statusbar.