Without providing a specific slot, QEMU won't be able to create the
second additional PCIe root port with the following error:

    $ qemu-system-x86_64 [...] -machine q35 \
    >    -device pcie-root-port,bus=pcie.0,id=rp1 \
    >    -device pcie-root-port,bus=pcie.0,id=rp2
    qemu-system-x86_64: -device pcie-root-port,bus=pcie.0,id=rp2:
    Can't add chassis slot, error -16

This is due to the fact they both try to use slot 0. Update the
documentation to specify a slot for each new PCIe root port.

Signed-off-by: Vincent Bernat <vincent@bernat.ch>
Message-Id: <20210614114357.1146725-1-vincent@bernat.ch>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

according to Eduardo Habkost's commit fd3b02c8 all PCIEs now implement
INTERFACE_PCIE_DEVICE so we don't need is_express field anymore.

Devices that implements only INTERFACE_PCIE_DEVICE (is_express == 1)
or
devices that implements only INTERFACE_CONVENTIONAL_PCI_DEVICE (is_express == 0)
where not affected by the change.

The only devices that were affected are those that are hybrid and also
had (is_express == 1) - therefor only:
  - hw/vfio/pci.c
  - hw/usb/hcd-xhci.c
  - hw/xen/xen_pt.c

For those 3 I made sure that QEMU_PCI_CAP_EXPRESS is on in instance_init()

Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Yoni Bettan <ybettan@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Signed-off-by: Aleksandr Bezzubikov <zuban32s@gmail.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Tested-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>