xen/acpi: Disable ACPI memory hotplug when running under Xen.

xen/acpi: Disable ACPI memory hotplug when running under Xen.

This came about as we back-ported f5775e0b6116b7e2425ccf535243b21768566d87
"x86/xen: discard RAM regions above the maximum reservation".

where on certain very specific Intel platforms (which can do memory
hotplug) we would crash during bootup.

The reason for the failure is that we use the baremetal mechanism
to do memory hotplug - which is not appropiate. We should use
the xen-acpi-memory.c one, but ..

config XEN_ACPI_HOTPLUG_MEMORY

    tristate "Xen ACPI memory hotplug"

    depends on XEN_DOM0 && XEN_STUB && ACPI
and
config XEN_STUB

    bool "Xen stub drivers"

    depends on XEN && X86_64 && BROKEN

See the 'BROKEN'? That came about
76fc253723add627cf28c09c79fb67e71f9e4782
"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.

   ..
Fixing 76fc253723add627cf28c09c79fb67e71f9e4782 would be nice, but
that is some upstream work to get done and we don't have the time.

In the meantime we can:
 1) Revert f5775e0b6116b7e2425ccf535243b21768566d87
    I would prefer not as we would diverge from upstream when it comes
    to backporting features.
 2) Simulate acpi_no_memhotplug being passed on the Linux command line.
    That is much easier, and we can carry this patch until upstream
    gets a proper fix.

The patch "solves" the regression be implementing 2).

Reported-and-Tested-by: Deepak Patel <deepak.patel@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
OraBug:23735125