pc: pcihp: expose MMIO base and len as properties

it will be used later to dynamically reserve MMIO region
instead of manually punching holes in PCI0._CRS

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
