Move HVM's device_model to a hypervisor parameter

This moves yet another hardcoded value to a hypervisor parameter. I
removed the 64/32 difference as it doesn't seem valid to me - it's more
of a local site config rather than arch config.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>