[Qemu-ppc] Why is "spapr-pci-host-bridge" no-user?

[Markus Armbruster]

Commit 09aa9a5 "spapr-pci: enable adding PHB via -device" set
cannot_instantiate_with_device_add_yet without also adding a comment
explaining why.  It is currently the only one lacking such a comment.
Let's fix that.

Unfortunately, the commit message doesn't tell me (or I'm too dense to
understand it):

    spapr-pci: enable adding PHB via -device
    
    Recent changes introduced cannot_instantiate_with_device_add_yet
    and removed capability of adding yet another PCI host bridge via
    command line for SPAPR platform (POWERPC64 server).
    
    This brings the capability back and puts SPAPR PHB into "bridge"
    category.
    
    This is not much use for emulated PHB but it is absolutely required
    for VFIO as we put an IOMMU group onto a separate PHB on SPAPR.
    
    Signed-off-by: Alexey Kardashevskiy <address@hidden>
    Signed-off-by: Alexander Graf <address@hidden>

Alexey, can you explain why the device cannot be used with -device /
device_add?