hw/sparc64/niagara: use int64_t for vdisk size to avoid truncation

blk_getlength() returns int64_t, but niagara_init() stored it in an int,
truncating the if=pflash virtual-ramdisk size for images >= 2 GiB. A ~4 GiB
image truncated to 0/negative, failed the `size > 0` check, and exit(1)'d
before the CPU ran, ending with:

  qemu-system-sparc64: could not load ram disk

Signed-off-by: Dmitry Pimenov <sun4qemu@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Message-ID: <20260710222350.9185-1-sun4qemu@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>