SUPPORT: downgrade Physical CPU Hotplug to Experimental

The feature is not commonly used, and we don't have hardware to test it,
not in OSSTest, not in Gitlab, and not even ad-hoc manually by community
members. We could use QEMU to test it, but even that it is known not to
work on our end.

Also take the opportunity to rename the feature to "ACPI CPU Hotplug"
for clarity.

Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

CHANGELOG.md

@@ -13,6 +13,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
    known user doesn't use it properly, leading to in-guest breakage.
  - The "dom0" option is now supported on Arm and "sve=" sub-option can be used
    to enable dom0 guest to use SVE/SVE2 instructions.
+ - Physical CPU Hotplug downgraded to Experimental and renamed "ACPI CPU
+   Hotplug" for clarity
 
 ### Added
  - On x86, support for features new in Intel Sapphire Rapids CPUs:

SUPPORT.md

@@ -46,9 +46,9 @@ For the Cortex A77 r0p0 - r1p0, see Errata 1508412.
 
 ## Host hardware support
 
-### Physical CPU Hotplug
+### ACPI CPU Hotplug
 
-    Status, x86: Supported
+    Status, x86: Experimental
 
 ### Physical Memory