docs: Update arch doc for one proxy / VM
docs: Update arch doc for one proxy / VM

Update the architecture document to reflect the new proxy behaviour
where there is a proxy instance per virtual machine rather than a
single host-level proxy instance.

This should strictly have been handled on #835.

Fixes #908.

Signed-off-by: James O. D. Hunt <348fbaadfbada55ea105602bf5609e23ac1c4898@intel.com>

clearcontainers/runtime,clearcontainers/runtime,devimc/runtime,chavafg/runtime,devimc/runtime,chavafg/runtime