CVE-2020-2732

[Thadeu Lima de Souza Cascardo]

These are backports of the fixes to CVE-2020-2732. They affect X86 hosts. A
nested guest (L2) may execute disallowed code, leading to access to L1
resources.

I used the backport from 4.4.y, which includes "emulate RDPID" as a pre-req, so
we divert the least as possible from stable upstream.

I did the same for Bionic 4.15, using the backports from 4.14.y.

D/E/F have been built-tested, Ben Romer did some nested guest smoke testing
with the Bionic patches, although with a version without the pre-req.

Xenial is build testing right now. More tests are in development.