AMD OSVW (OS Visible Workaround) for Xen

AMD OSVW (OS Visible Workaround) for Xen

This path enables AMD OSVW (OS Visible Workaround) feature for
Xen. New AMD errata will have a OSVW id assigned in the future. OS is
supposed to check OSVW status MSR to find out whether CPU has a
specific erratum. Legacy errata are also supported in this patch:
traditional family/model/stepping approach will be used if OSVW
feature isn't applicable.  This patch is adapted from Hans Rosenfeld's
patch submitted to Linux kernel.

Signed-off-by: Wei Huang <wei.huang2@amd.com>
Signed-off-by: Hans Rosenfeld <hands.rosenfeld@amd.com>
Acked-by: Jan Beulich <jbeulich@novell.com>