virtio-blk: surface krun_add_disk2 block_id as device ID/serial

VIRTIO_BLK_T_GET_ID previously returned a file-derived image id, so
guests saw arbitrary serials under /sys/block/*/serial and could not
locate disks by the block_id the host passed to krun_add_disk2. Use
the supplied block id as the device ID so exact-serial discovery works.

Assisted-by: Codex / GPT-5
Signed-off-by: Ryan Dunn <dunnry@hotmail.com>