pc: Fix disabling of vapic for compat PC models

We used to be able to address both the QEMU and the KVM APIC via "apic".
This doesn't work anymore. So we need to use their parent class to turn
off the vapic on machines that should not expose them.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>