m25p80: Improve error when the backend file size does not match the device

m25p80: Improve error when the backend file size does not match the device

Currently, when a block backend is attached to a m25p80 device and the
associated file size does not match the flash model, QEMU complains
with the error message "failed to read the initial flash content".
This is confusing for the user.

Instead, use helper blk_check_size_and_read_all() introduced by commit
06f1521795 ("pflash: Require backend size to match device, improve
errors").

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Delevoryas <peter@pjd.dev>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221115151000.2080833-1-clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>