flask/policy: fix static device labeling examples

flask/policy: fix static device labeling examples

The definitions of static device labels must be placed at the end of the
policy.conf before passing it to checkpolicy; the existing examples
(which are commented out) are in the wrong location.  Create a new file
for device contexts which will place them in the proper location.

This also removes some directions about using the xen policy type in
checkpolicy which is no longer needed.

Reported-by: Julien Grall <julien.grall@linaro.org>
Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Ian Campbell <ian.campbell@citrix.com>