cpu: Partially revert "cpu: Change qemu_init_vcpu() argument to CPUState"

Commit c643bed99 moved qemu_init_vcpu() calls to common CPUState code.
This causes x86 cpu-add to fail with "KVM: setting VAPIC address failed".

The reason for the failure is that CPUClass::kvm_fd is not yet
initialized in the following call graph:
->x86_cpu_realizefn
 ->x86_cpu_apic_realize
  ->qdev_init
   ->device_set_realized
    ->device_reset (hotplugged == 1)
     ->apic_reset_common
      ->vapic_base_update
       ->kvm_apic_vapic_base_update
This causes attempted KVM vCPU ioctls to fail.

By contrast, in the non-hotplug case the APIC is reset much later, when
the vCPU is already initialized.

As a quick and safe solution, move the qemu_init_vcpu() call back into
the targets' realize functions.

Reported-by: Chen Fan <chen.fan.fnst@cn.fujitsu.com>
Acked-by: Igor Mammedov <imammedo@redhat.com> (for i386)
Signed-off-by: Andreas Färber <afaerber@suse.de>