pnv/phb4: Delete unused "pnv-phb4-pec-stack" devices

pnv/phb4: Delete unused "pnv-phb4-pec-stack" devices

The number of stacks is controlled by property "num-stacks".
pnv_pec_instance_init() creates the maximum supported number, because
the property has not been set then.  pnv_pec_realize() realizes only
the wanted number.  Works, although it can leave unrealized devices
hanging around in the QOM composition tree.  Affects machine powernv9.

Delete the unused devices by making pnv_pec_realize() unparent them.
Visible in "info qom-tree":

     /machine (powernv9-machine)
       /chip[0] (power9_v2.0-pnv-chip)
         [...]
         /pec[0] (pnv-phb4-pec)
           /stack[0] (pnv-phb4-pec-stack)
             [...]
    -      /stack[1] (pnv-phb4-pec-stack)
    -        /phb (pnv-phb4)
    -          /pcie-mmcfg-mmio[0] (qemu:memory-region)
    -          /root (pnv-phb4-root-port)
    -          /source (xive-source)
    -      /stack[2] (pnv-phb4-pec-stack)
    -        /phb (pnv-phb4)
    -          /pcie-mmcfg-mmio[0] (qemu:memory-region)
    -          /root (pnv-phb4-root-port)
    -          /source (xive-source)
           /xscom-pec-0.0-nest[0] (qemu:memory-region)
           /xscom-pec-0.0-pci[0] (qemu:memory-region)
         /pec[1] (pnv-phb4-pec)
           /stack[0] (pnv-phb4-pec-stack)
             [...]
           /stack[1] (pnv-phb4-pec-stack)
             [...]
    -      /stack[2] (pnv-phb4-pec-stack)
    -        /phb (pnv-phb4)
    -          /pcie-mmcfg-mmio[0] (qemu:memory-region)
    -          /root (pnv-phb4-root-port)
    -          /source (xive-source)
           /xscom-pec-0.1-nest[0] (qemu:memory-region)
           /xscom-pec-0.1-pci[0] (qemu:memory-region)
         /pec[2] (pnv-phb4-pec)
           /stack[0] (pnv-phb4-pec-stack)
             [...]
           /stack[1] (pnv-phb4-pec-stack)
             [...]
           /stack[2] (pnv-phb4-pec-stack)
             [...]

Cc: Cédric Le Goater <clg@kaod.org>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: qemu-ppc@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20200609122339.937862-12-armbru@redhat.com>