Convert the acpi memory hotplug spec to rST.

Note that this includes converting a lot of weird whitespace
characters to plain old spaces (the rST parser does not like
whatever the old ones were).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-id: 20210727170414.3368-3-peter.maydell@linaro.org