target/ppc: Fix setting of cpu->compat_pvr on incoming migration

cpu->compat_pvr is used to store the current compat mode of the cpu.

On the receiving side during incoming migration we check compatibility
with the compat mode by calling ppc_set_compat(). However we fail to set
the compat mode with the hypervisor since the "new" compat mode doesn't
differ from the current (due to a "cpu->compat_pvr != compat_pvr" check).
This means that kvm runs the vcpus without a compat mode, which is the
incorrect behaviour. The implication being that a compatibility mode
will never be in effect after migration.

To fix this so that the compat mode is correctly set with the
hypervisor, store the desired compat mode and reset cpu->compat_pvr to
zero before calling ppc_set_compat().

Fixes: 5dfaa532 ("ppc: fix ppc_set_compat() with KVM PR")

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
