machine: Move smp_prefer_sockets to struct SMPCompatProps

Now we have a common structure SMPCompatProps used to store information
about SMP compatibility stuff, so we can also move smp_prefer_sockets
there for cleaner code.

No functional change intended.

Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210929025816.21076-15-wangyanan55@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>