block/io: Use QEMU_LOCK_GUARD to simplify mutex handling Replace qemu_mutex_(un)lock() calls with QEMU_LOCK_GUARD(). Signed-off-by: Evgeny Kolmakov <randomjack94dev@gmail.com> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org> Reviewed-by: Zhang Chen <zhangckid@gmail.com> Reviewed-by: Michael Tokarev <mjt@tls.msk.ru> Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>