kernel: xen: hypercalls exposed to privilege rings 1 and 2 of HVM guests

Xen 3.0.3 through 4.1.x possibly 4.1.6.1, 4.2.x possibly 4.2.3, and 4.3.x possibly 4.3.1 does not properly prevent access to hypercalls, which allows local guest users to gain privileges via a crafted application running in ring 1 or 2...