xen/acpi-stub: Disable it b/c the acpi_processor_add is no longer called.

With the Xen ACPI stub code (CONFIG_XEN_STUB=y) enabled, the power
C and P states are no longer uploaded to the hypervisor.

The reason is that the Xen CPU hotplug code: xen-acpi-cpuhotplug.c
and the xen-acpi-stub.c register themselves as the "processor" type object.

That means the generic processor (processor_driver.c) stops
working and it does not call (acpi_processor_add) which populates the

         per_cpu(processors, pr->id) = pr;

structure. The 'pr' is gathered from the acpi_processor_get_info function
which does the job of finding the C-states and figuring out PBLK address.

The 'processors->pr' is then later used by xen-acpi-processor.c (the one that
uploads C and P states to the hypervisor). Since it is NULL, we end
skip the gathering of _PSD, _PSS, _PCT, etc and never upload the power
management data.

The end result is that enabling the CONFIG_XEN_STUB in the build means that
xen-acpi-processor is not working anymore.

This temporary patch fixes it by marking the XEN_STUB driver as
BROKEN until this can be properly fixed.

CC: jinsong.liu@intel.com
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>