dump/win_dump: add 32-bit guest Windows support

Before this patch, 'dump-guest-memory -w' was accepting only 64-bit
dump header provided by guest through vmcoreinfo and thus was unable
to produce 32-bit guest Windows dump. So, add 32-bit guest Windows
dumping support.

Signed-off-by: Viktor Prutyanov <viktor.prutyanov@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
[ misc error handling fixes to avoid compiler warning ]
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20220406171558.199263-5-viktor.prutyanov@redhat.com>