It is currently not possible to manipulate EFI variables from within a running guest. For example, setting the boot-args variable by running the nvram tool in a Mac OS X guest will not work. As an alternative method, VBoxInternal2/EfiBootArgs extradata can be passed to a VM in order to set the boot-args variable. To change the boot-args EFI variable, use the following command:

VBoxManage setextradata "VM name" VBoxInternal2/EfiBootArgs <value>