OvmfPkg/CpuHotplugSmm: collect hot-unplug events

OvmfPkg/CpuHotplugSmm: collect hot-unplug events

Process fw_remove events in QemuCpuhpCollectApicIds(), and collect APIC IDs
and QEMU CPU Selectors for CPUs being hot-unplugged.

In addition, we now ignore CPUs which only have remove set. These
CPUs haven't been processed by OSPM yet.

This is based on the QEMU hot-unplug protocol documented here:
  https://lore.kernel.org/qemu-devel/20201204170939.1815522-3-imammedo@redhat.com/

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Aaron Young <aaron.young@oracle.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3132
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
Message-Id: <20210312062656.2477515-3-ankur.a.arora@oracle.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>