target-ppc: Fix kvmppc_set_compat to use negotiated cpu-version

By mistake, QEMU uses the maximum compatibility level from the command
line instead of the value negotiated in client-architecture-support call.

This replaces @max_compat with @cpu_version. This only affects guests
which do not support the host CPU.

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