doc: live migration missing part about different cpu models/flags

[do3meli]

- [X] This is a doc addition request.

The current live migration documentation [1] and the corresponding configuration guide [2] do not mention any word about the cpu model and flags requirements that are needed in order to successfully live migrate an instance.

I believe that it is a crucial piece of information for any openstack administrator to know what prerequisites are needed for it to complete.

[1] https://docs.openstack.org/nova/latest/admin/live-migration-usage.html
[2] https://docs.openstack.org/nova/latest/admin/configuring-migrations.html#section-configuring-compute-migrations

[Matt Riedemann (mriedem)]

Agree. Unfortunately I don't have the best words available to describe this in the docs.