[libvirt] [PATCH] virt-aa-helper should require <uuid> in XML

When <uuid> is not in the XML, a virUUIDGenerate() ends up being called which is unnecessary and can lead to crashes if /dev/urandom isn't available because virRandomInitialize() is not called within virt-aa-helper. This patch adds verify_xpath_context() and updates caps_mockup() to use it. This is https://launchpad.net/bugs/672943 -- Jamie Strandboge | http://www.canonical.com