tools: remove fs-front/fs-back

tools: remove fs-front/fs-back

Its access controls are really not OK.  In particular, it's not good for
libxl, which stores per-VM config blobs in a directory that is exported
to all VMs.

This will break stub-qemu save/restore, which is the only user of
fs-front that I'm aware of, but:
 - It's currently broken anyway (fs-back isn't run by default and crashes
   if it is run manually); and
 - Stefano has a plan to plumb qemu save records through a dedicated
   console channel instead.

Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>