OvmfPkg/CpuHotplugSmm: refactor hotplug logic

OvmfPkg/CpuHotplugSmm: refactor hotplug logic

Refactor CpuHotplugMmi() to pull out the CPU hotplug logic into
ProcessHotAddedCpus(). This is in preparation for supporting CPU
hot-unplug.

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>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210312062656.2477515-2-ankur.a.arora@oracle.com>