[Xen-API] Extension of the Xen-API for managing Xen Security Policies

[Xen-API] Extension of the Xen-API for managing Xen Security Policies

This patch implements extensions for managing security policies in
xend. The XSPolicy and ACMPolicy classes provide the interface for the
Xen-API and implement functionality for setting, updating and
activating of a Xen security policy as well as labeling of virtual
machines and resources such as block devices. Labeling of network
devices will follow.

The acmpolicy class implements a compiler for translating an XML
policy into their binary format and provides functionality for
comparing a current policy against a new one when changing/updating a
policy.

The xspolicyadmin class administers the policy of the system.

Some of the xend-internal code deals with transforming the labeling
information from the S-Expression format into the new Xen-API
format. This is similar to much of the other code that is already
there.

Signed-off-by: Stefan Berger <stefanb@us.ibm.com>