docs: update ACPI CPU hotplug spec with new protocol

docs: update ACPI CPU hotplug spec with new protocol

Add description of new CPU hotplug interface.

To switch from from legacy mode into new mode use fact
that write accesses into CPU present bitmap were never
used before and were ignored by QEMU.
So use it to as a way to switch from legacy mode.
That way pc/q35 machine starts in legacy mode and
QEMU generated ACPI tables will switch to new CPU
hotplug interface during runtime.
In case QEMU is started with legacy BIOS (that doesn't
support QEMU generated ACPI tables), legacy CPU hotplug
will remain active and could be used by BIOS built in
ACPI tables for CPU hotplug.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>