PCIe features are available only via the 'q35' machine type for x86 and
the 'virt' machine type for AArch64 architecture.

Mention that explicitly.

Thanks: Daniel Berrangé

Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>