spapr: Make spapr_cpu_core_unrealize() idempotent

spapr: Make spapr_cpu_core_unrealize() idempotent

spapr_cpu_core_realize() has a rollback path which partially duplicates
the code of spapr_cpu_core_unrealize().

Let's make spapr_cpu_core_unrealize() idempotent and call it instead. This
requires to:
- move the registration and unregistration of the reset handler around
  but it is harmless,
- allocate the array of vCPUs with g_new0() to be able to filter out
  unused slots,
- make sure to only unrealize vCPUs that have been already realized.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <160279672626.1808373.14142129300586424514.stgit@bahia.lan>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>