OvmfPkg/CpuHotplugSmm: do actual CPU hot-eject

OvmfPkg/CpuHotplugSmm: do actual CPU hot-eject

Add logic in EjectCpu() to do the actual the CPU ejection.

On the BSP, ejection happens by first selecting the CPU via
its QemuSelector and then sending the QEMU "eject" command.
QEMU in-turn signals the remote VCPU thread which context-switches
the CPU out of the SMI handler.

Meanwhile the CPU being ejected, waits around in its holding
area until it is context-switched out. Note that it is possible
that a slow CPU gets ejected before it reaches the wait loop.
However, this would never happen before it has executed the
"AllCpusInSync" loop in SmiRendezvous().
It can mean that an ejected CPU does not execute code after
that point but given that the CPU state will be destroyed by
QEMU, the missed cleanup is no great loss.

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: <20210312062656.2477515-10-ankur.a.arora@oracle.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
[lersek@redhat.com: unneeded inner QemuSelector declaration in EjectCpu()
 triggers VS warning #4456 (local variable shadowed); remove it]