spapr: Don't use QOM [*] syntax for DR connectors.

The dynamic reconfiguration (hotplug) code for the pseries machine type
uses a "DR connector" QOM object for each resource it will be possible
to hotplug.  Each of these is added to its owner using
    object_property_add_child(owner, "dr-connector[*], ...);

That works ok, mostly, but it means that the property indices are
arbitrary, depending on the order in which the connectors are constructed.
That might line up to something useful, but it doesn't have to.

It will get worse once we add hotplug RAM support.  That will add a DR
connector object for every 256MB of potential memory.  So if maxmem=2T,
for example, there are 8192 objects under the same parent.

The QOM interfaces aren't really designed for this.  In particular
object_property_add() with [*] has O(n^2) time complexity (in the number of
existing children): first it has a linear search through array indices to
find a free slot, each of which is attempted to a recursive call to
object_property_add() with a specific [N].  Those calls are O(n) because
there's a linear search through all properties to check for duplicates.

By using a meaningful index value, which we already know is unique we can
avoid the [*] special behaviour.  That lets us reduce the total time for
creating the DR objects from O(n^3) to O(n^2).

O(n^2) is still kind of crappy, but it's enough to reduce the startup time
of qemu (with in-progress memory hotplug support) with maxmem=2T from ~20
minutes to ~4 seconds.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Cc: Bharata B Rao <bharata@linux.vnet.ibm.com>
Tested-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
