PVHVM VCPU hotplug mechanism via ACPI hotplug doesn't work in Xen 4.[1, 2, 3]

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

Which is probably the case b/c the Linux side implementation for
such simple operation as :

echo 0 > /sys/devices/system/cpu/cpu1/online
echo 1 > /sys/devices/system/cpu/cpu1/online

would have blown up so nobody tested it?

Now that is fixed (v3.10 + http://lists.xen.org/archives/html/xen-devel/2013-05/msg00503.html)
I tried to do 'xm vcpu-set latest X' for a PVHVM guest.

The first iteration was using this simple guest config:
builder='hvm'
disk = [ 'file:/mnt/lab/latest/root_image.iso,hdc:cdrom,r']
memory = 2048
boot="d"
maxvcpus=4
vcpus=4
serial='pty'
vnclisten="0.0.0.0"
name="latest"
vif = [ 'mac=00:0F:4B:00:00:68, bridge=switch' ]

And I tried simple combinations of 'x[m|l] vcpu-set latest 2|3|4' and
none of them worked.

In Xen 4.1 (and Xen 4.3 if I use:device_model_version="qemu-xen-traditional")
I saw that the qemu log does the right thing:
.. snip..
Remove vcpu 2
Remove vcpu 1

and the guest's ACPI SCI is incrementing:
# cat /proc/interrupts  |grep acpi
   9:          1          0          0   IO-APIC-fasteoi   acpi
# cat /proc/interrupts  |grep acpi
   9:          2          0          0   IO-APIC-fasteoi   acpi

But nothing looks to be happening. Where should I look?
The ACPI DSDT is a bit daunting. Has this ever worked in the past?
If so, what code ran to hotplug CPUs?