OvmfPkg/CpuHotplugSmm: add function for collecting CPUs with events

OvmfPkg/CpuHotplugSmm: add function for collecting CPUs with events

Add a function that collects the APIC IDs of CPUs that have just been
hot-plugged, or are about to be hot-unplugged.

Pending events are only located and never cleared; QEMU's AML needs the
firmware to leave the status bits intact in the hotplug register block.

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: <20200226221156.29589-10-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Tested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>