Plumbing for this is in progress. Subtasks include:
1) Refactoring vm_launcher support to handle managing export IDs
2) Creating/managing VM sandbox directories on the host (/home/root/<blah>/vm-data/<vm name>
3) Authenticating NFS via IP only, and adding iptables rules on the host to blackhole VM traffic if a VM tries to change its IP.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform2/+/469e9dfd572a82916f67f44d677a0f5b1a4b4195

commit 469e9dfd572a82916f67f44d677a0f5b1a4b4195
Author: Stephen Barber <smbarber@chromium.org>
Date: Tue Oct 03 03:25:24 2017

vm_tools: launcher: refactor nfs_launcher

One instance of ganesha can be shared among multiple VMs, but
each VM will be restricted to a single sandbox directory on
the host. Each sandbox directory will receive its own export,
which will be accessible to a single VM.

BUG= chromium:703938 
TEST=launch VM and observe the joys of NFS

Change-Id: I39d976569dbac58372ddd7e6e639b04edf36aedc
Reviewed-on: https://chromium-review.googlesource.com/679920
Commit-Ready: Stephen Barber <smbarber@chromium.org>
Tested-by: Stephen Barber <smbarber@chromium.org>
Reviewed-by: Stephen Barber <smbarber@chromium.org>

[modify] https://crrev.com/469e9dfd572a82916f67f44d677a0f5b1a4b4195/vm_tools/host.gypi
[delete] https://crrev.com/01c22576c5c7c5090f8c5ec774fe354efb6e8e06/vm_tools/launcher/nfs_launcher.h
[modify] https://crrev.com/469e9dfd572a82916f67f44d677a0f5b1a4b4195/vm_tools/launcher/subnet.h
[modify] https://crrev.com/469e9dfd572a82916f67f44d677a0f5b1a4b4195/vm_tools/launcher/vm_launcher.cc
[delete] https://crrev.com/01c22576c5c7c5090f8c5ec774fe354efb6e8e06/vm_tools/launcher/nfs_launcher.cc
[modify] https://crrev.com/469e9dfd572a82916f67f44d677a0f5b1a4b4195/vm_tools/launcher/crosvm.h
[add] https://crrev.com/469e9dfd572a82916f67f44d677a0f5b1a4b4195/vm_tools/launcher/nfs_export.cc
[add] https://crrev.com/469e9dfd572a82916f67f44d677a0f5b1a4b4195/vm_tools/launcher/nfs_export.h
[modify] https://crrev.com/469e9dfd572a82916f67f44d677a0f5b1a4b4195/vm_tools/launcher/crosvm.cc
[modify] https://crrev.com/469e9dfd572a82916f67f44d677a0f5b1a4b4195/vm_tools/launcher/subnet.cc