spapr_drc: Prevent detach racing against attach for CPU DR

If a CPU is hot removed while hotplug of the same is still in progress,
the guest crashes. Prevent this by ensuring that detach is done only
after attach has completed.

The existing code already prevents such race for PCI hotplug. However
given that CPU is a logical DR unlike PCI and starts with ISOLATED
state, we need a logic that works for CPU too.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
               [Don't set awaiting_attach for PCI devices]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
