vdpa: Convert sprintf/snprintf to sysfs_emit

Per filesystems/sysfs.rst, show() should only use sysfs_emit()
or sysfs_emit_at() when formatting the value to be returned to user space.

coccinelle complains that there are still a couple of functions that use
snprintf(). Convert them to sysfs_emit().

sprintf() will be converted as weel if they have.

Generally, this patch is generated by
make coccicheck M=<path/to/file> MODE=patch \
COCCI=scripts/coccinelle/api/device_attr_show.cocci

No functional change intended

CC: "Michael S. Tsirkin" <mst@redhat.com>
CC: Jason Wang <jasowang@redhat.com>
CC: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
CC: virtualization@lists.linux.dev
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Message-Id: <20240314095853.1326111-1-lizhijian@fujitsu.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>