OvmfPkg/CpuHotplugSmm: introduce skeleton for CPU Hotplug SMM driver

OvmfPkg/CpuHotplugSmm: introduce skeleton for CPU Hotplug SMM driver

Add a new SMM driver skeleton that registers a root SMI handler, and
checks if the SMI control value (written to 0xB2) indicates a CPU hotplug
SMI.

QEMU's ACPI payload will cause the OS to raise a broadcast SMI when a CPU
hotplug event occurs, namely by writing value 4 to IO Port 0xB2. In other
words, control value 4 is now allocated for this purpose; introduce the
ICH9_APM_CNT_CPU_HOTPLUG macro for it.

The standard identifiers in this driver use the new MM (Management Mode)
terminology from the PI spec, not the earlier SMM (System Management Mode)
terms.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1512
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200226221156.29589-7-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>