virtio: Implementing attribute show with sysfs_emit

Replace sprintf with sysfs_emit or its variants for their
built-in PAGE_SIZE awareness.

Signed-off-by: Dawei Li <set_pte_at@outlook.com>
Message-Id: <TYCP286MB23232A999FE7DBDF50BA0FAACA0F9@TYCP286MB2323.JPNP286.PROD.OUTLOOK.COM>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>