Installing GRUB on a VirtualBox guest - does it affect the host MBR?
Quoted from http://www.virtualbox.org/manual/ch05.html#vdidetails
Disk image files reside on the host system and are seen by the guest systems as hard disks of a certain geometry. When a guest operating system reads from or writes to a hard disk, VirtualBox redirects the request to the image file.
So it does not affect the host MBR. Only the disk image file.