docs: pcie: Spell out machine type needs for PCIe features

docs: pcie: Spell out machine type needs for PCIe features

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>