docs: add slot when adding new PCIe root port

docs: add slot when adding new PCIe root port

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>