hw/pflash: implement update buffer for block writes

hw/pflash: implement update buffer for block writes

Add an update buffer where all block updates are staged.
Flush or discard updates properly, so we should never see
half-completed block writes in pflash storage.

Drop a bunch of FIXME comments ;)

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240108160900.104835-4-kraxel@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
(cherry picked from commit 284a7ee2e290e0c9b8cd3ea6164d92386933054f)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
(Mjt: drop const in hw/block/pflash_cfi01.c for before
 v8.2.0-220-g7d5dc0a367 "hw/block: Constify VMState")