hw/pci: Add pcie_count_ds_port() and pcie_find_port_first() helpers

hw/pci: Add pcie_count_ds_port() and pcie_find_port_first() helpers

These two helpers enable host bridges to operate differently depending on
the number of downstream ports, in particular if there is only a single
port.

Useful for CXL where HDM address decoders are allowed to be implicit in
the host bridge if there is only a single root port.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20230227153128.8164-2-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>