target-ppc: Remove PVR check from migration

Currently migration fails if CPU version (PVR register) is different
even a bit. This check is performed at the very end of migration when
device states are sent. This is too late for management software and
we need to provide a way for the user to make sure that migration
will succeed if QEMU is started with appropritate command line parameters.

This removes the PVR check.

This resets PVR to the default value as the existing VMSTATE record
for SPR array sends all 1024 registers unconditionally and overwrites
the destination PVR.

If the user wants some guarantees for migration to succeed, then
a CPU name or "host" CPU with a "compat" option (on its way to upsteam)
should be used and KVM or TCG is expected to fail on unsupported values
at the moment of QEMU start.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
