[other] importing a directory tree into a VM

is there any VM implementation, yet, that can import a directory tree from the host instead of a virtualized block device, and not via NFS? i would expect this will also need a new file system type on the guest OS (i'm intending to only run a variety of Linux things this way ... no Windows for me) to see the file system. i am wanting to import the guest root (/) file system this way.