xen: information leak through outs instruction emulation (XSA-67)

The outs instruction emulation in Xen 3.1.x, 4.2.x, 4.3.x, and earlier, when using FS: or GS: segment override, uses an uninitialized variable as a segment base, which allows local 64-bit PV guests to obtain sensitive information hypervisor stack content via unspecified vectors related to stale...