docs: add guidance on configuring CPU models for x86

With the recent set of CPU hardware vulnerabilities on x86, it is
increasingly difficult to understand which CPU configurations are
good to use and what flaws they might be vulnerable to.

This doc attempts to help management applications and administrators in
picking sensible CPU configuration on x86 hosts. It outlines which of
the named CPU models are good choices, and describes which extra CPU
flags should be enabled to allow the guest to mitigate hardware flaws.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20180627160103.13634-1-berrange@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>