xen_machine_pv: do not create a dummy CPU in machine->init

This fixes a regression introduced by:

commit 62fc403f11523169eb4264de31279745f48e3ecc
Author: Igor Mammedov <imammedo@redhat.com>
Date:   Mon Apr 29 18:54:13 2013 +0200

    target-i386: Attach ICC bus to CPU on its creation

    X86CPU should have parent bus so it could provide bus for child APIC.

The commit makes it mandatory to pass a valid ICC bus to cpu_x86_create,
but cpu_x86_init just passes NULL to it.
xen_machine_pv uses cpu_x86_init, therefore it has been broken.

This patch fixes the problem by removing the dummy CPU creation
altogether from xen_init_pv, relying on the fact that QEMU can now cope
with a machine without an emulated CPU.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
CC: imammedo@redhat.com
CC: qemu-stable@nongnu.org
