cc-check: Document "unrestricted_guest" requirement
cc-check: Document "unrestricted_guest" requirement

The `cc-check` command requires the "unrestricted_guest" KVM feature.
This is used as a heuristic to determine if the system is new enough to
run a Clear Container.

Fixes #421.

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

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