hw/core/loader: return image sizes as ssize_t

hw/core/loader: return image sizes as ssize_t

Various loader functions return an int which limits images to 2GB which
is fine for things like a BIOS/kernel image, but if we want to be able
to load memory images or large ramdisks then any file over 2GB would
silently fail to load.

Cc: Luc Michel <lmichel@kalray.eu>
Signed-off-by: Jamie Iles <jamie@nuviainc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Luc Michel <lmichel@kalray.eu>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211111141141.3295094-2-jamie@nuviainc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>