There is no option in VirtualBox to hide Menubar and Statusbar. Many people likes to hide them so that they can get maximum space in their guest OS. Virtual Box provides
VBoxManage command. It can do all the things that VirtualBox GUI can do. Furthermore it can help us in getting rid of Menubar and Statusbar.
Here is a nice example of other VBoxManage commands.
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.