spapr: fix default DRC state for coldplugged LMBs

Currently we set the initial isolation/allocation state for DRCs
associated with coldplugged LMBs to ISOLATED/UNUSABLE,
respectively, under the assumption that the guest will move this
state to UNISOLATED/USABLE.

In fact, this is only the case for LMBs added via hotplug. For
coldplugged LMBs, the guest actually assumes the initial state to
be UNISOLATED/USABLE.

In practice, this only becomes an issue when we attempt to unplug
one of these LMBs, where the guest kernel will issue an
rtas-get-sensor-state call to check that the corresponding DRC is
in an USABLE state before it will release the LMB back to
QEMU. If the returned state is otherwise, the guest will assume no
further action is needed, which bypasses the QEMU-side cleanup that
occurs during the USABLE->UNUSABLE transition. This results in
LMBs and their corresponding pc-dimm devices to stick around
indefinitely.

This patch fixes the issue by manually setting DRCs associated with
cold-plugged LMBs to UNISOLATED/ALLOCATED, but leaving the hotplug
state untouched. As it turns out, this is analogous to the handling
for cold-plugged CPUs in spapr_core_plug().

Cc: qemu-ppc@nongnu.org
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Bharata B Rao <bharata@linux.vnet.ibm.com>
Cc: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
