A maneira mais fácil e rápida seria criar uma pasta compartilhada :
With the "shared folders" feature of VirtualBox, you can access files of your host system from within the guest system. This is similar how you would use network shares in Windows networks -- except that shared folders do not need require networking, only the Guest Additions.
Shared folders must physically reside on the host and are then shared with the guest
To share a host folder with a virtual machine in VirtualBox, you must specify the path of that folder and choose for it a "share name" that the guest can use to access it. Hence, first create the shared folder on the host; then, within the guest, connect to it.
In the window of a running VM, you can select "Shared folders" from the "Devices" menu, or click on the folder icon on the status bar in the bottom right corner.
Basta seguir a documentação do VirtualBox.
É o caminho mais rápido porque não requer nenhum serviço de rede "real" entre eles (como o FTP, como você mencionou), ou até mesmo o uso do Dropbox (que é muito lento para esse propósito, embora seja mais fácil de configurar).