xsm: Add support for Xen device policies

xsm: Add support for Xen device policies

Add support for Xen ocontext records to enable device polices.  The
default policy will not be changed and instructions have been added to
enable the new functionality.  Examples on how to use the new policy
language have been added but commented out.  The newest version of
checkpolicy (>= 2.0.20) and libsepol (>= 2.0.39) is needed in order to
compile it.  Devices can be labeled and enforced using the following
new commands; pirqcon, iomemcon, ioportcon and pcidevicecon.

Signed-off-by : George Coker <gscoker@alpha.ncsc.mil>
Signed-off-by : Paul Nuzzi <pjnuzzi@tycho.ncsc.mil>