conf: Add hvf domain type

conf: Add hvf domain type

QEMU supports Hypervisor.framework since 2.12 as hvf accel.
Hypervisor.framework provides a lightweight interface to run a virtual
cpu on macOS without the need to install third-party kernel
extensions (KEXTs).

It's supported since macOS 10.10 on machines with Intel VT-x feature
set that includes Extended Page Tables (EPT) and Unrestricted Mode.

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>