spapr_cpu_core: vmstate_[un]register per-CPU data from (un)realizefn

spapr_cpu_core: vmstate_[un]register per-CPU data from (un)realizefn

VMStateDescription vmstate_spapr_cpu_state was added by commit
b94020268e0b6 (spapr_cpu_core: migrate per-CPU data) to migrate per-CPU
data with the required vmstate registration and unregistration calls.
However the unregistration is being done only from vcpu creation error path
and not from CPU delete path.

This causes migration to fail with the following error if migration is
attempted after a CPU unplug like this:
Unknown savevm section or instance 'spapr_cpu' 16
Additionally this leaves the source VM unresponsive after migration failure.

Fix this by ensuring the vmstate_unregister happens during CPU removal.
Fixing this becomes easier when vmstate (un)registration calls are moved to
vcpu (un)realize functions which is what this patch does.

Fixes: https://bugs.launchpad.net/qemu/+bug/1785972
Reported-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Signed-off-by: Bharata B Rao <bharata@linux.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
(cherry picked from commit cc71c7760e263f808c4240a725425671eeeb7e4d)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>