docs: Improve documentation for CPU topology

docs: Improve documentation for CPU topology

On the guest configuration side, mention that support for the
"dies" attribute was introduced in libvirt 6.1.0 and clarify
that the ability to use non-default values is subject to
architecture and machine limitations.

On the host capabilities side, the documentation was pretty
much entirely missing. It's still far from perfect, but anything
is better than having no information at all.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>