hw/pcie: Provide a utility function for control of EP / SW USP link

Whilst similar to existing PCIESlot link configuration a few registers
need to be set differently so that the downstream device presents
a 'configured' state that is then used to 'train' the upstream port
on the link.  Basically that means setting the status register to
reflect it succeeding in training up to target settings.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20240916173518.1843023-5-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>