cpu-kvm, arch-x86: Fix KVM on Intel platforms

cpu-kvm, arch-x86: Fix KVM on Intel platforms

This is the minimal set of changes from the patch that's been floating
around for a few years originally by Mike Upton.

See http://reviews.gem5.org/r/2613/ and
https://gem5-review.googlesource.com/c/public/gem5/+/7361

The change to the tssDesc is the minimal change to get KVM working on
Intel platforms. However, the other changes seem prudent to add.

Tested on both Intel (i7-7700) and AMD (EPYC 7451) platforms.

Change-Id: I000c7ba102ba161c2bb5e224bf826216cf0ff87a
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/12278
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>