docs: Add hvf on QEMU driver page

docs: Add hvf on QEMU driver page

It's worth to make the domain type a little bit more visible than a row
in news. An example of hvf domain is available on QEMU driver page.

While at it, mention Hypervisor.framework on index page.

Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Tested-by: Brad Laue <brad@brad-x.com>
Tested-by: Christophe Fergeau <cfergeau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>